Expose namespace storage statistics with GraphQL

Root namespaces have storage statistics.
This commit allows namespace owners to get those stats via GraphQL
queries like the following one

{
  namespace(fullPath: "a_namespace_path") {
    rootStorageStatistics {
      storageSize
      repositorySize
      lfsObjectsSize
      buildArtifactsSize
      packagesSize
      wikiSize
    }
  }
}

2 files changed, 15 insertions, 1 deletions

diff --git a/spec/graphql/types/namespace_type_spec.rb b/spec/graphql/types/namespace_type_spec.rb
index e1153832cc9..f476dd7286f 100644
--- a/spec/graphql/types/namespace_type_spec.rb
+++ b/spec/graphql/types/namespace_type_spec.rb
@@ -8,7 +8,7 @@ describe GitlabSchema.types['Namespace'] do
   it 'has the expected fields' do
     expected_fields = %w[
       id name path full_name full_path description description_html visibility
-      lfs_enabled request_access_enabled projects
+      lfs_enabled request_access_enabled projects root_storage_statistics
     ]
 
     is_expected.to have_graphql_fields(*expected_fields)
diff --git a/spec/graphql/types/root_storage_statistics_type_spec.rb b/spec/graphql/types/root_storage_statistics_type_spec.rb
new file mode 100644
index 00000000000..8c69c13aa73
--- /dev/null
+++ b/spec/graphql/types/root_storage_statistics_type_spec.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe GitlabSchema.types['RootStorageStatistics'] do
+  it { expect(described_class.graphql_name).to eq('RootStorageStatistics') }
+
+  it 'has all the required fields' do
+    is_expected.to have_graphql_fields(:storage_size, :repository_size, :lfs_objects_size,
+                                       :build_artifacts_size, :packages_size, :wiki_size)
+  end
+
+  it { is_expected.to require_graphql_authorizations(:read_statistics) }
+end