Add support for JSON logging for audit events

This will add audit_json.log that writes one line per audit event. For
example:

{
       "severity":"INFO",
       "time":"2018-10-17T17:38:22.523Z",
       "author_id":3,
       "entity_id":2,
       "entity_type":"Project",
       "change":"visibility",
       "from":"Private",
       "to":"Public",
       "author_name":"John Doe4",
       "target_id":2,
       "target_type":"Project",
       "target_details":"namespace2/project2"
}

1 files changed, 9 insertions, 0 deletions

diff --git a/lib/gitlab/audit_json_logger.rb b/lib/gitlab/audit_json_logger.rb
new file mode 100644
index 00000000000..12e0645f3e4
--- /dev/null
+++ b/lib/gitlab/audit_json_logger.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Gitlab
+  class AuditJsonLogger < Gitlab::JsonLogger
+    def self.file_name_noext
+      'audit_json'
+    end
+  end
+end