Add "action" tag to /internal/allowed API

This allows us to re-use any other analytics that rely on the "action"
tag having a value set.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.7.0 (unreleased)
+  - Transactions for /internal/allowed now have an "action" tag set
   - Method instrumentation now uses Module#prepend instead of aliasing methods
   - Repository.clean_old_archives is now instrumented
   - Add support for environment variables on a job level in CI configuration file

lib/api/internal.rb

@@ -23,6 +23,8 @@ module API
       end
 
       post "/allowed" do
+        Gitlab::Metrics.tag_transaction('action', 'Grape#/internal/allowed')
+
         status 200
 
         actor =