Enable prometheus metrics by default

changelogs/unreleased/issue_43660.yml

+---
+title: Enable prometheus monitoring by default
+merge_request:
+author:
+type: other

config/initializers/8_metrics.rb

@@ -119,7 +119,14 @@ def instrument_classes(instrumentation)
 end
 # rubocop:enable Metrics/AbcSize
 
-if Gitlab::Metrics.enabled?
+# With prometheus enabled by default this breaks all specs
+# that stubs methods using `any_instance_of` for the models reloaded here.
+#
+# We should deprecate the usage of `any_instance_of` in the future
+# check: https://github.com/rspec/rspec-mocks#settings-mocks-or-stubs-on-any-instance-of-a-class
+#
+# Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/33587
+if Gitlab::Metrics.enabled? && !Rails.env.test?
   require 'pathname'
   require 'influxdb'
   require 'connection_pool'

db/migrate/20180503200320_enable_prometheus_metrics_by_default.rb

+class EnablePrometheusMetricsByDefault < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    change_column_default :application_settings, :prometheus_metrics_enabled, true
+  end
+
+  def down
+    change_column_default :application_settings, :prometheus_metrics_enabled, false
+  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: 20180503175054) do
+ActiveRecord::Schema.define(version: 20180503200320) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -134,7 +134,7 @@ ActiveRecord::Schema.define(version: 20180503175054) do
     t.integer "cached_markdown_version"
     t.boolean "clientside_sentry_enabled", default: false, null: false
     t.string "clientside_sentry_dsn"
-    t.boolean "prometheus_metrics_enabled", default: false, null: false
+    t.boolean "prometheus_metrics_enabled", default: true, null: false
     t.boolean "help_page_hide_commercial_content", default: false
     t.string "help_page_support_url"
     t.integer "performance_bar_allowed_group_id"

spec/db/production/settings_spec.rb

@@ -48,15 +48,15 @@ describe 'seed production settings' do
       end
     end
 
-    context 'GITLAB_PROMETHEUS_METRICS_ENABLED is false' do
+    context 'GITLAB_PROMETHEUS_METRICS_ENABLED is default' do
       before do
         stub_env('GITLAB_PROMETHEUS_METRICS_ENABLED', '')
       end
 
-      it 'prometheus_metrics_enabled is set to false' do
+      it 'prometheus_metrics_enabled is set to true' do
         load(settings_file)
 
-        expect(settings.prometheus_metrics_enabled).to eq(false)
+        expect(settings.prometheus_metrics_enabled).to eq(true)
       end
     end
   end