Use feature flag instead of application settigns to control if method calls should be instrumented

app/helpers/application_settings_helper.rb

@@ -211,7 +211,6 @@ module ApplicationSettingsHelper
       :polling_interval_multiplier,
       :project_export_enabled,
       :prometheus_metrics_enabled,
-      :prometheus_metrics_method_instrumentation_enabled,
       :recaptcha_enabled,
       :recaptcha_private_key,
       :recaptcha_site_key,

app/views/admin/application_settings/_form.html.haml

@@ -361,14 +361,6 @@
               %code prometheus_multiproc_dir
               does not exist or is not pointing to a valid directory.
               = link_to icon('question-circle'), help_page_path('administration/monitoring/prometheus/gitlab_metrics', anchor: 'metrics-shared-directory')
-    .form-group
-      .col-sm-offset-2.col-sm-10
-        .checkbox
-          = f.label :prometheus_metrics_method_instrumentation_enabled do
-            = f.check_box :prometheus_metrics_method_instrumentation_enabled
-            Track method execution time.
-            .help-block
-              Provides method execution metrics. Can adversely impact GitLab's responsiveness.
 
   %fieldset
     %legend Profiling - Performance Bar

db/migrate/20171122211913_add_prometheus_instrumentation_to_application_settings.rb

-class AddPrometheusInstrumentationToApplicationSettings < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME = false
-  disable_ddl_transaction!
-
-  def up
-    add_column_with_default(:application_settings, :prometheus_metrics_method_instrumentation_enabled, :boolean,
-                            default: false, allow_null: false)
-  end
-
-  def down
-    remove_column(:application_settings, :prometheus_metrics_method_instrumentation_enabled)
-  end
-end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171122211913) do
+ActiveRecord::Schema.define(version: 20171121144800) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -149,7 +149,6 @@ ActiveRecord::Schema.define(version: 20171122211913) do
     t.boolean "throttle_authenticated_web_enabled", default: false, null: false
     t.integer "throttle_authenticated_web_requests_per_period", default: 7200, null: false
     t.integer "throttle_authenticated_web_period_in_seconds", default: 3600, null: false
-    t.boolean "prometheus_metrics_method_instrumentation_enabled", default: false, null: false
   end
 
   create_table "audit_events", force: :cascade do |t|

lib/api/settings.rb

@@ -66,9 +66,6 @@ module API
       optional :max_pages_size, type: Integer, desc: 'Maximum size of pages in MB'
       optional :container_registry_token_expire_delay, type: Integer, desc: 'Authorization token duration (minutes)'
       optional :prometheus_metrics_enabled, type: Boolean, desc: 'Enable Prometheus metrics'
-      given prometheus_metrics_enabled: ->(val) { val } do
-        requires :prometheus_metrics_method_instrumentation_enabled, type: Boolean, desc: 'Enable method call instrumentation'
-      end
       optional :metrics_enabled, type: Boolean, desc: 'Enable the InfluxDB metrics'
       given metrics_enabled: ->(val) { val } do
         requires :metrics_host, type: String, desc: 'The InfluxDB host'

lib/gitlab/metrics/method_call.rb

@@ -45,7 +45,7 @@ module Gitlab
         @cpu_time += cpu_time
         @call_count += 1
 
-        if prometheus_enabled? && above_threshold?
+        if call_measurement_enabled? && above_threshold?
           self.class.call_duration_histogram.observe(@transaction.labels.merge(labels), real_time / 1000.0)
         end
 
@@ -71,8 +71,8 @@ module Gitlab
         real_time >= Metrics.method_call_threshold
       end
 
-      def prometheus_enabled?
-        @prometheus_enabled ||= Gitlab::CurrentSettings.current_application_settings[:prometheus_metrics_method_instrumentation_enabled]
+      def call_measurement_enabled?
+        Feature.get(:prometheus_metrics_method_instrumentation).enabled?
       end
     end
   end

spec/lib/gitlab/metrics/method_call_spec.rb

@@ -20,8 +20,7 @@ describe Gitlab::Metrics::MethodCall do
 
       context 'prometheus instrumentation is enabled' do
         before do
-          allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
-                                              .and_return(prometheus_metrics_method_instrumentation_enabled: true)
+          Feature.get(:prometheus_metrics_method_instrumentation).enable
         end
 
         it 'observes the performance of the supplied block' do
@@ -35,8 +34,7 @@ describe Gitlab::Metrics::MethodCall do
 
       context 'prometheus instrumentation is disabled' do
         before do
-          allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
-                                              .and_return(prometheus_metrics_method_instrumentation_enabled: false)
+          Feature.get(:prometheus_metrics_method_instrumentation).disable
         end
 
         it 'does not observe the performance' do
@@ -52,8 +50,7 @@ describe Gitlab::Metrics::MethodCall do
       before do
         allow(method_call).to receive(:above_threshold?).and_return(false)
 
-        allow(Gitlab::CurrentSettings).to receive(:current_application_settings)
-                                            .and_return(prometheus_metrics_method_instrumentation_enabled: true)
+        Feature.get(:prometheus_metrics_method_instrumentation).enable
       end
 
       it 'does not observe the performance' do