Handle InvalidForeignKey error within `ensure_metrics`

What does this MR do?

This MR fixes a race condition that could lead to InvalidForeignKey error.

Example:

Merge request is being processed in two different processes (process_1, process_2).

1. process_2 calls save on an existing MR.
2. process_1 calls delete on the merge request at the same time.
3. process_2 starts processing after_save callbacks
4. process_1 finished deleting the MR.
5. process_2 has no metrics records (deleted) and it tries to create it (#ensure_metrics)
6. process_2 fails to create the metrics record since the merge_request record is not there anymore (invalid FK)

I'm setting the ~backstage label because the error mostly occurs within sidekiq.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #200095 (closed)