Take two for MR metrics population background migration

What does this MR do?

This is a "take two" for running the background migration which populates MR metrics with events data.

Why was this MR needed?

The initial batch, which was scheduled in 10.4 did not completely migrate all records and left 400_000 MRs without a metrics association. If my memory does not fail me, we paused the background migration processing a few times on this release, which probably led to data being lost on Redis, but I'm not entirely sure on that.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Part of #41587 (closed)