Storing of application metrics in InfluxDB

This adds the ability to write application metrics (e.g. SQL timings) to
InfluxDB. These metrics can in turn be visualized using Grafana, or
really anything else that can read from InfluxDB. These metrics can be
used to track application performance over time, between different Ruby
versions, different GitLab versions, etc.

== Transaction Metrics

Currently the following is tracked on a per transaction basis (a
transaction is a Rails request or a single Sidekiq job):

* Timings per query along with the raw (obfuscated) SQL and information
  about what file the query originated from.
* Timings per view along with the path of the view and information about
  what file triggered the rendering process.
* The duration of a request itself along with the controller/worker
  class and method name.
* The duration of any instrumented method calls (more below).

== Sampled Metrics

Certain metrics can't be directly associated with a transaction. For
example, a process' total memory usage is unrelated to any running
transactions. While a transaction can result in the memory usage going
up there's no accurate way to determine what transaction is to blame,
this becomes especially problematic in multi-threaded environments.

To solve this problem there's a separate thread that takes samples at a
fixed interval. This thread (using the class Gitlab::Metrics::Sampler)
currently tracks the following:

* The process' total memory usage.
* The number of file descriptors opened by the process.
* The amount of Ruby objects (using ObjectSpace.count_objects).
* GC statistics such as timings, heap slots, etc.

The default/current interval is 15 seconds, any smaller interval might
put too much pressure on InfluxDB (especially when running dozens of
processes).

== Method Instrumentation

While currently not yet used methods can be instrumented to track how
long they take to run. Unlike the likes of New Relic this doesn't
require modifying the source code (e.g. including modules), it all
happens from the outside. For example, to track `User.by_login` we'd add
the following code somewhere in an initializer:

    Gitlab::Metrics::Instrumentation.
      instrument_method(User, :by_login)

to instead instrument an instance method:

    Gitlab::Metrics::Instrumentation.
      instrument_instance_method(User, :save)

Instrumentation for either all public model methods or a few crucial
ones will be added in the near future, I simply haven't gotten to doing
so just yet.

== Configuration

By default metrics are disabled. This means users don't have to bother
setting anything up if they don't want to. Metrics can be enabled by
editing one's gitlab.yml configuration file (see
config/gitlab.yml.example for example settings).

== Writing Data To InfluxDB

Because InfluxDB is still a fairly young product I expect the worse.
Data loss, unexpected reboots, the database not responding, you name it.
Because of this data is _not_ written to InfluxDB directly, instead it's
queued and processed by Sidekiq. This ensures that users won't notice
anything when InfluxDB is giving trouble.

The metrics worker can be started in a standalone manner as following:

    bundle exec sidekiq -q metrics

The corresponding class is called MetricsWorker.

13 files changed, 678 insertions, 0 deletions

diff --git a/spec/lib/gitlab/metrics/delta_spec.rb b/spec/lib/gitlab/metrics/delta_spec.rb
new file mode 100644
index 00000000000..718387cdee1
--- /dev/null
+++ b/spec/lib/gitlab/metrics/delta_spec.rb
@@ -0,0 +1,16 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Delta do
+  let(:delta) { described_class.new }
+
+  describe '#compared_with' do
+    it 'returns the delta as a Numeric' do
+      expect(delta.compared_with(5)).to eq(5)
+    end
+
+    it 'bases the delta on a previously used value' do
+      expect(delta.compared_with(5)).to eq(5)
+      expect(delta.compared_with(15)).to eq(10)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/instrumentation_spec.rb b/spec/lib/gitlab/metrics/instrumentation_spec.rb
new file mode 100644
index 00000000000..eb31c9e52cd
--- /dev/null
+++ b/spec/lib/gitlab/metrics/instrumentation_spec.rb
@@ -0,0 +1,91 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Instrumentation do
+  before do
+    @dummy = Class.new do
+      def self.foo(text = 'foo')
+        text
+      end
+
+      def bar(text = 'bar')
+        text
+      end
+    end
+  end
+
+  describe '.instrument_method' do
+    describe 'with metrics enabled' do
+      before do
+        allow(Gitlab::Metrics).to receive(:enabled?).and_return(true)
+
+        Gitlab::Metrics::Instrumentation.instrument_method(@dummy, :foo)
+      end
+
+      it 'renames the original method' do
+        expect(@dummy).to respond_to(:_original_foo)
+      end
+
+      it 'calls the instrumented method with the correct arguments' do
+        expect(@dummy.foo).to eq('foo')
+      end
+
+      it 'fires an ActiveSupport notification upon calling the method' do
+        expect(ActiveSupport::Notifications).to receive(:instrument).
+          with('class_method.method_call', module: @dummy, name: :foo)
+
+        @dummy.foo
+      end
+    end
+
+    describe 'with metrics disabled' do
+      before do
+        allow(Gitlab::Metrics).to receive(:enabled?).and_return(false)
+      end
+
+      it 'does not instrument the method' do
+        Gitlab::Metrics::Instrumentation.instrument_method(@dummy, :foo)
+
+        expect(@dummy).to_not respond_to(:_original_foo)
+      end
+    end
+  end
+
+  describe '.instrument_instance_method' do
+    describe 'with metrics enabled' do
+      before do
+        allow(Gitlab::Metrics).to receive(:enabled?).and_return(true)
+
+        Gitlab::Metrics::Instrumentation.
+          instrument_instance_method(@dummy, :bar)
+      end
+
+      it 'renames the original method' do
+        expect(@dummy.method_defined?(:_original_bar)).to eq(true)
+      end
+
+      it 'calls the instrumented method with the correct arguments' do
+        expect(@dummy.new.bar).to eq('bar')
+      end
+
+      it 'fires an ActiveSupport notification upon calling the method' do
+        expect(ActiveSupport::Notifications).to receive(:instrument).
+          with('instance_method.method_call', module: @dummy, name: :bar)
+
+        @dummy.new.bar
+      end
+    end
+
+    describe 'with metrics disabled' do
+      before do
+        allow(Gitlab::Metrics).to receive(:enabled?).and_return(false)
+      end
+
+      it 'does not instrument the method' do
+        Gitlab::Metrics::Instrumentation.
+          instrument_instance_method(@dummy, :bar)
+
+        expect(@dummy.method_defined?(:_original_bar)).to eq(false)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/metric_spec.rb b/spec/lib/gitlab/metrics/metric_spec.rb
new file mode 100644
index 00000000000..ec39bc9cce8
--- /dev/null
+++ b/spec/lib/gitlab/metrics/metric_spec.rb
@@ -0,0 +1,57 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Metric do
+  let(:metric) do
+    described_class.new('foo', { number: 10 }, { host: 'localtoast' })
+  end
+
+  describe '#series' do
+    subject { metric.series }
+
+    it { is_expected.to eq('foo') }
+  end
+
+  describe '#values' do
+    subject { metric.values }
+
+    it { is_expected.to eq({ number: 10 }) }
+  end
+
+  describe '#tags' do
+    subject { metric.tags }
+
+    it { is_expected.to eq({ host: 'localtoast' }) }
+  end
+
+  describe '#to_hash' do
+    it 'returns a Hash' do
+      expect(metric.to_hash).to be_an_instance_of(Hash)
+    end
+
+    describe 'the returned Hash' do
+      let(:hash) { metric.to_hash }
+
+      it 'includes the series' do
+        expect(hash[:series]).to eq('foo')
+      end
+
+      it 'includes the tags' do
+        expect(hash[:tags]).to be_an_instance_of(Hash)
+
+        expect(hash[:tags][:hostname]).to be_an_instance_of(String)
+        expect(hash[:tags][:ruby_engine]).to be_an_instance_of(String)
+        expect(hash[:tags][:ruby_version]).to be_an_instance_of(String)
+        expect(hash[:tags][:gitlab_version]).to be_an_instance_of(String)
+        expect(hash[:tags][:process_type]).to be_an_instance_of(String)
+      end
+
+      it 'includes the values' do
+        expect(hash[:values]).to eq({ number: 10 })
+      end
+
+      it 'includes the timestamp' do
+        expect(hash[:timestamp]).to be_an_instance_of(Fixnum)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/obfuscated_sql_spec.rb b/spec/lib/gitlab/metrics/obfuscated_sql_spec.rb
new file mode 100644
index 00000000000..6e9b62016d6
--- /dev/null
+++ b/spec/lib/gitlab/metrics/obfuscated_sql_spec.rb
@@ -0,0 +1,79 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::ObfuscatedSQL do
+  describe '#to_s' do
+    describe 'using single values' do
+      it 'replaces a single integer' do
+        sql = described_class.new('SELECT x FROM y WHERE a = 10')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+      end
+
+      it 'replaces a single float' do
+        sql = described_class.new('SELECT x FROM y WHERE a = 10.5')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+      end
+
+      it 'replaces a single quoted string' do
+        sql = described_class.new("SELECT x FROM y WHERE a = 'foo'")
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+      end
+
+      if Gitlab::Database.mysql?
+        it 'replaces a double quoted string' do
+          sql = described_class.new('SELECT x FROM y WHERE a = "foo"')
+
+          expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+        end
+      end
+
+      it 'replaces a single regular expression' do
+        sql = described_class.new('SELECT x FROM y WHERE a = /foo/')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+      end
+
+      it 'replaces regular expressions using escaped slashes' do
+        sql = described_class.new('SELECT x FROM y WHERE a = /foo\/bar/')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE a = ?')
+      end
+    end
+
+    describe 'using consecutive values' do
+      it 'replaces multiple integers' do
+        sql = described_class.new('SELECT x FROM y WHERE z IN (10, 20, 30)')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE z IN (3 values)')
+      end
+
+      it 'replaces multiple floats' do
+        sql = described_class.new('SELECT x FROM y WHERE z IN (1.5, 2.5, 3.5)')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE z IN (3 values)')
+      end
+
+      it 'replaces multiple single quoted strings' do
+        sql = described_class.new("SELECT x FROM y WHERE z IN ('foo', 'bar')")
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE z IN (2 values)')
+      end
+
+      if Gitlab::Database.mysql?
+        it 'replaces multiple double quoted strings' do
+          sql = described_class.new('SELECT x FROM y WHERE z IN ("foo", "bar")')
+
+          expect(sql.to_s).to eq('SELECT x FROM y WHERE z IN (2 values)')
+        end
+      end
+
+      it 'replaces multiple regular expressions' do
+        sql = described_class.new('SELECT x FROM y WHERE z IN (/foo/, /bar/)')
+
+        expect(sql.to_s).to eq('SELECT x FROM y WHERE z IN (2 values)')
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/rack_middleware_spec.rb b/spec/lib/gitlab/metrics/rack_middleware_spec.rb
new file mode 100644
index 00000000000..a143fe4cfcd
--- /dev/null
+++ b/spec/lib/gitlab/metrics/rack_middleware_spec.rb
@@ -0,0 +1,63 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::RackMiddleware do
+  let(:app) { double(:app) }
+
+  let(:middleware) { described_class.new(app) }
+
+  let(:env) { { 'REQUEST_METHOD' => 'GET', 'REQUEST_URI' => '/foo' } }
+
+  describe '#call' do
+    before do
+      expect_any_instance_of(Gitlab::Metrics::Transaction).to receive(:finish)
+    end
+
+    it 'tracks a transaction' do
+      expect(app).to receive(:call).with(env).and_return('yay')
+
+      expect(middleware.call(env)).to eq('yay')
+    end
+
+    it 'tags a transaction with the name and action of a controller' do
+      klass      = double(:klass, name: 'TestController')
+      controller = double(:controller, class: klass, action_name: 'show')
+
+      env['action_controller.instance'] = controller
+
+      allow(app).to receive(:call).with(env)
+
+      expect(middleware).to receive(:tag_controller).
+        with(an_instance_of(Gitlab::Metrics::Transaction), env)
+
+      middleware.call(env)
+    end
+  end
+
+  describe '#transaction_from_env' do
+    let(:transaction) { middleware.transaction_from_env(env) }
+
+    it 'returns a Transaction' do
+      expect(transaction).to be_an_instance_of(Gitlab::Metrics::Transaction)
+    end
+
+    it 'tags the transaction with the request method and URI' do
+      expect(transaction.tags[:request_method]).to eq('GET')
+      expect(transaction.tags[:request_uri]).to eq('/foo')
+    end
+  end
+
+  describe '#tag_controller' do
+    let(:transaction) { middleware.transaction_from_env(env) }
+
+    it 'tags a transaction with the name and action of a controller' do
+      klass      = double(:klass, name: 'TestController')
+      controller = double(:controller, class: klass, action_name: 'show')
+
+      env['action_controller.instance'] = controller
+
+      middleware.tag_controller(transaction, env)
+
+      expect(transaction.tags[:action]).to eq('TestController#show')
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/sampler_spec.rb b/spec/lib/gitlab/metrics/sampler_spec.rb
new file mode 100644
index 00000000000..b486d38870f
--- /dev/null
+++ b/spec/lib/gitlab/metrics/sampler_spec.rb
@@ -0,0 +1,92 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Sampler do
+  let(:sampler) { described_class.new(5) }
+
+  describe '#start' do
+    it 'gathers a sample at a given interval' do
+      expect(sampler).to receive(:sleep).with(5)
+      expect(sampler).to receive(:sample)
+      expect(sampler).to receive(:loop).and_yield
+
+      sampler.start.join
+    end
+  end
+
+  describe '#sample' do
+    it 'samples various statistics' do
+      expect(sampler).to receive(:sample_memory_usage)
+      expect(sampler).to receive(:sample_file_descriptors)
+      expect(sampler).to receive(:sample_objects)
+      expect(sampler).to receive(:sample_gc)
+      expect(sampler).to receive(:flush)
+
+      sampler.sample
+    end
+
+    it 'clears any GC profiles' do
+      expect(sampler).to receive(:flush)
+      expect(GC::Profiler).to receive(:clear)
+
+      sampler.sample
+    end
+  end
+
+  describe '#flush' do
+    it 'schedules the metrics using Sidekiq' do
+      expect(MetricsWorker).to receive(:perform_async).
+        with([an_instance_of(Hash)])
+
+      sampler.sample_memory_usage
+      sampler.flush
+    end
+  end
+
+  describe '#sample_memory_usage' do
+    it 'adds a metric containing the memory usage' do
+      expect(Gitlab::Metrics::System).to receive(:memory_usage).
+        and_return(9000)
+
+      expect(Gitlab::Metrics::Metric).to receive(:new).
+        with('memory_usage', value: 9000).
+        and_call_original
+
+      sampler.sample_memory_usage
+    end
+  end
+
+  describe '#sample_file_descriptors' do
+    it 'adds a metric containing the amount of open file descriptors' do
+      expect(Gitlab::Metrics::System).to receive(:file_descriptor_count).
+        and_return(4)
+
+      expect(Gitlab::Metrics::Metric).to receive(:new).
+        with('file_descriptors', value: 4).
+        and_call_original
+
+      sampler.sample_file_descriptors
+    end
+  end
+
+  describe '#sample_objects' do
+    it 'adds a metric containing the amount of allocated objects' do
+      expect(Gitlab::Metrics::Metric).to receive(:new).
+        with('object_counts', an_instance_of(Hash)).
+        and_call_original
+
+      sampler.sample_objects
+    end
+  end
+
+  describe '#sample_gc' do
+    it 'adds a metric containing garbage collection statistics' do
+      expect(GC::Profiler).to receive(:total_time).and_return(0.24)
+
+      expect(Gitlab::Metrics::Metric).to receive(:new).
+        with('gc_statistics', an_instance_of(Hash)).
+        and_call_original
+
+      sampler.sample_gc
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/sidekiq_middleware_spec.rb b/spec/lib/gitlab/metrics/sidekiq_middleware_spec.rb
new file mode 100644
index 00000000000..05214efc565
--- /dev/null
+++ b/spec/lib/gitlab/metrics/sidekiq_middleware_spec.rb
@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::SidekiqMiddleware do
+  let(:middleware) { described_class.new }
+
+  describe '#call' do
+    it 'tracks the transaction' do
+      worker = Class.new.new
+
+      expect_any_instance_of(Gitlab::Metrics::Transaction).to receive(:finish)
+
+      middleware.call(worker, 'test', :test) { nil }
+    end
+
+    it 'does not track jobs of the MetricsWorker' do
+      worker = MetricsWorker.new
+
+      expect(Gitlab::Metrics::Transaction).to_not receive(:new)
+
+      middleware.call(worker, 'test', :test) { nil }
+    end
+  end
+
+  describe '#tag_worker' do
+    it 'adds the worker class and action to the transaction' do
+      trans  = Gitlab::Metrics::Transaction.new
+      worker = double(:worker, class: double(:class, name: 'TestWorker'))
+
+      expect(trans).to receive(:add_tag).with(:action, 'TestWorker#perform')
+
+      middleware.tag_worker(trans, worker)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/subscribers/action_view_spec.rb b/spec/lib/gitlab/metrics/subscribers/action_view_spec.rb
new file mode 100644
index 00000000000..77f3e69d523
--- /dev/null
+++ b/spec/lib/gitlab/metrics/subscribers/action_view_spec.rb
@@ -0,0 +1,32 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Subscribers::ActionView do
+  let(:transaction) { Gitlab::Metrics::Transaction.new }
+
+  let(:subscriber) { described_class.new }
+
+  let(:event) do
+    root = Rails.root.to_s
+
+    double(:event, duration: 2.1,
+                   payload:  { identifier: "#{root}/app/views/x.html.haml" })
+  end
+
+  before do
+    allow(subscriber).to receive(:current_transaction).and_return(transaction)
+
+    allow(Gitlab::Metrics).to receive(:last_relative_application_frame).
+      and_return(['app/views/x.html.haml', 4])
+  end
+
+  describe '#render_template' do
+    it 'tracks rendering of a template' do
+      values = { duration: 2.1, file: 'app/views/x.html.haml', line: 4 }
+
+      expect(transaction).to receive(:add_metric).
+        with(described_class::SERIES, values, path: 'app/views/x.html.haml')
+
+      subscriber.render_template(event)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/subscribers/active_record_spec.rb b/spec/lib/gitlab/metrics/subscribers/active_record_spec.rb
new file mode 100644
index 00000000000..58e8e84df9b
--- /dev/null
+++ b/spec/lib/gitlab/metrics/subscribers/active_record_spec.rb
@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Subscribers::ActiveRecord do
+  let(:transaction) { Gitlab::Metrics::Transaction.new }
+
+  let(:subscriber) { described_class.new }
+
+  let(:event) do
+    double(:event, duration: 0.2,
+                   payload:  { sql: 'SELECT * FROM users WHERE id = 10' })
+  end
+
+  before do
+    allow(subscriber).to receive(:current_transaction).and_return(transaction)
+
+    allow(Gitlab::Metrics).to receive(:last_relative_application_frame).
+      and_return(['app/models/foo.rb', 4])
+  end
+
+  describe '#sql' do
+    it 'tracks the execution of a SQL query' do
+      values = { duration: 0.2, file: 'app/models/foo.rb', line: 4 }
+      sql    = 'SELECT * FROM users WHERE id = ?'
+
+      expect(transaction).to receive(:add_metric).
+        with(described_class::SERIES, values, sql: sql)
+
+      subscriber.sql(event)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/subscribers/method_call_spec.rb b/spec/lib/gitlab/metrics/subscribers/method_call_spec.rb
new file mode 100644
index 00000000000..65de95d6d1e
--- /dev/null
+++ b/spec/lib/gitlab/metrics/subscribers/method_call_spec.rb
@@ -0,0 +1,41 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Subscribers::MethodCall do
+  let(:transaction) { Gitlab::Metrics::Transaction.new }
+
+  let(:subscriber) { described_class.new }
+
+  let(:event) do
+    double(:event, duration: 0.2,
+                   payload:  { module: double(:mod, name: 'Foo'), name: :foo })
+  end
+
+  before do
+    allow(subscriber).to receive(:current_transaction).and_return(transaction)
+
+    allow(Gitlab::Metrics).to receive(:last_relative_application_frame).
+      and_return(['app/models/foo.rb', 4])
+  end
+
+  describe '#instance_method' do
+    it 'tracks the execution of an instance method' do
+      values = { duration: 0.2, file: 'app/models/foo.rb', line: 4 }
+
+      expect(transaction).to receive(:add_metric).
+        with(described_class::SERIES, values, method: 'Foo#foo')
+
+      subscriber.instance_method(event)
+    end
+  end
+
+  describe '#class_method' do
+    it 'tracks the execution of a class method' do
+      values = { duration: 0.2, file: 'app/models/foo.rb', line: 4 }
+
+      expect(transaction).to receive(:add_metric).
+        with(described_class::SERIES, values, method: 'Foo.foo')
+
+      subscriber.class_method(event)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/system_spec.rb b/spec/lib/gitlab/metrics/system_spec.rb
new file mode 100644
index 00000000000..f8c1d956ca1
--- /dev/null
+++ b/spec/lib/gitlab/metrics/system_spec.rb
@@ -0,0 +1,29 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::System do
+  if File.exist?('/proc')
+    describe '.memory_usage' do
+      it "returns the process' memory usage in bytes" do
+        expect(described_class.memory_usage).to be > 0
+      end
+    end
+
+    describe '.file_descriptor_count' do
+      it 'returns the amount of open file descriptors' do
+        expect(described_class.file_descriptor_count).to be > 0
+      end
+    end
+  else
+    describe '.memory_usage' do
+      it 'returns 0.0' do
+        expect(described_class.memory_usage).to eq(0.0)
+      end
+    end
+
+    describe '.file_descriptor_count' do
+      it 'returns 0' do
+        expect(described_class.file_descriptor_count).to eq(0)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics/transaction_spec.rb b/spec/lib/gitlab/metrics/transaction_spec.rb
new file mode 100644
index 00000000000..5f17ff8ee75
--- /dev/null
+++ b/spec/lib/gitlab/metrics/transaction_spec.rb
@@ -0,0 +1,77 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics::Transaction do
+  let(:transaction) { described_class.new }
+
+  describe '#duration' do
+    it 'returns the duration of a transaction in seconds' do
+      transaction.run { sleep(0.5) }
+
+      expect(transaction.duration).to be >= 0.5
+    end
+  end
+
+  describe '#run' do
+    it 'yields the supplied block' do
+      expect { |b| transaction.run(&b) }.to yield_control
+    end
+
+    it 'stores the transaction in the current thread' do
+      transaction.run do
+        expect(Thread.current[described_class::THREAD_KEY]).to eq(transaction)
+      end
+    end
+
+    it 'removes the transaction from the current thread upon completion' do
+      transaction.run { }
+
+      expect(Thread.current[described_class::THREAD_KEY]).to be_nil
+    end
+  end
+
+  describe '#add_metric' do
+    it 'adds a metric tagged with the transaction UUID' do
+      expect(Gitlab::Metrics::Metric).to receive(:new).
+        with('foo', { number: 10 }, { transaction_id: transaction.uuid })
+
+      transaction.add_metric('foo', number: 10)
+    end
+  end
+
+  describe '#add_tag' do
+    it 'adds a tag' do
+      transaction.add_tag(:foo, 'bar')
+
+      expect(transaction.tags).to eq({ foo: 'bar' })
+    end
+  end
+
+  describe '#finish' do
+    it 'tracks the transaction details and submits them to Sidekiq' do
+      expect(transaction).to receive(:track_self)
+      expect(transaction).to receive(:submit)
+
+      transaction.finish
+    end
+  end
+
+  describe '#track_self' do
+    it 'adds a metric for the transaction itself' do
+      expect(transaction).to receive(:add_metric).
+        with(described_class::SERIES, { duration: transaction.duration }, {})
+
+      transaction.track_self
+    end
+  end
+
+  describe '#submit' do
+    it 'submits the metrics to Sidekiq' do
+      transaction.track_self
+
+      expect(MetricsWorker).to receive(:perform_async).
+        with([an_instance_of(Hash)])
+
+      transaction.submit
+    end
+  end
+end
diff --git a/spec/lib/gitlab/metrics_spec.rb b/spec/lib/gitlab/metrics_spec.rb
new file mode 100644
index 00000000000..ebc69f8a75f
--- /dev/null
+++ b/spec/lib/gitlab/metrics_spec.rb
@@ -0,0 +1,36 @@
+require 'spec_helper'
+
+describe Gitlab::Metrics do
+  describe '.pool_size' do
+    it 'returns a Fixnum' do
+      expect(described_class.pool_size).to be_an_instance_of(Fixnum)
+    end
+  end
+
+  describe '.timeout' do
+    it 'returns a Fixnum' do
+      expect(described_class.timeout).to be_an_instance_of(Fixnum)
+    end
+  end
+
+  describe '.enabled?' do
+    it 'returns a boolean' do
+      expect([true, false].include?(described_class.enabled?)).to eq(true)
+    end
+  end
+
+  describe '.hostname' do
+    it 'returns a String containing the hostname' do
+      expect(described_class.hostname).to eq(Socket.gethostname)
+    end
+  end
+
+  describe '.last_relative_application_frame' do
+    it 'returns an Array containing a file path and line number' do
+      file, line = described_class.last_relative_application_frame
+
+      expect(line).to eq(30)
+      expect(file).to eq('spec/lib/gitlab/metrics_spec.rb')
+    end
+  end
+end