Investigate why certain merge requests don't have metrics

Problem to solve

@igor.drozdov commented in !71485 (comment 695565757):

The migration logic seems to be removed because the migration had been completed, I assume. But the fact that this merge request exists and fixes an issue when metrics don't exist for a merge request implies that either something went wrong on the migration (😓) or we have corrupted data in GDK (I've noticed that this issue has been reproduced in GDK).

Original migration issue: gitlab-foss#41587 (closed)

GDK missing metrics issue: #340406 (closed)

Proposed solution

Investigate why metrics don't exist for some merge request.

Additional notes

Once all merge requests have metrics we can remove the MergeRequestMetricsHelper added in !71485 (merged).