Ensure merge request metrics exist when updating them

What does this MR do and why?

Normally merge request metrics are created in the after_commit hook of a merge request. If for some reason the merge request metrics failed to create after creation, we need to make sure they are present here because the after_commit hook only fires in the update case after a merge request update has been committed to the database. However, during the update, the metrics themselves may be modified before that can happen.

Relates to #26911 (closed)

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