Fix autoload issue for "Gitlab::Metrics::Subscribers::ActiveRecord"

This commit fixes spring errors like:

A copy of Gitlab::Metrics::Subscribers::ActiveRecord has
been removed from the module tree but is still active!

lib/gitlab/metrics/subscribers/active_record.rb

@@ -61,7 +61,7 @@ module Gitlab
         end
 
         def current_transaction
-          Transaction.current
+          ::Gitlab::Metrics::Transaction.current
         end
       end
     end