Add metrics for keep-around ref creation

What does this MR do and why?

#415714

Keep-around refs are used to stop dangling commits from being garbage collected so that various UI elements on gitlab continue to work. The problem is that it is impossible to evaluate where these refs are created after the fact, all you know is that they got created. So here we add metrics that can be explicitly bucketed by the source of the keep-around.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Create a new merge request, add a review comment.
2. Check in http://127.0.0.1:3000/-/metrics for gitlab_keeparound_refs_created_total and gitlab_keeparound_refs_requested_total