Allow to enable query analyzers in production

This adds `GITLAB_ENABLE_QUERY_ANALYZERS` (disabled by default)
to allow enable this in production.

config/initializers/database_query_analyzers.rb

 # frozen_string_literal: true
 
 # Currently we register validator only for `dev` or `test` environment
-Gitlab::Database::QueryAnalyzer.new.hook! if Gitlab.dev_or_test_env?
+if Gitlab.dev_or_test_env? || Gitlab::Utils.to_boolean('GITLAB_ENABLE_QUERY_ANALYZERS', default: false)
+  Gitlab::Database::QueryAnalyzer.instance.hook!
+end

lib/gitlab/database/query_analyzer.rb

@@ -11,6 +11,10 @@ module Gitlab
         :sql, :connection, :pg
       )
 
+      def self.instance
+        @instance ||= new
+      end
+
       def hook!
         @subscriber = ActiveSupport::Notifications.subscribe('sql.active_record') do |event|
           process_sql(event.payload[:sql], event.payload[:connection])