Reduce the cardinality of GitLab metrics

What does this MR do?

We were producing million+ metrics across our fleet of web workers, which caused some trouble to Prometheus to handle all that data.

This MR that reduces the sizes of metrics buckets, and puts a few metrics behind feature flags. It also addresses some usuability issues as well by introducing API that simplifies using Prometheus metrics since old API combined with safely cached feature flags would add some code duplicates making it a bit more fragile.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #41771 (closed) https://gitlab.com/gitlab-org/gitlab-ce/issues/41407