Add merge and closed user ids to MR metrics

What does this MR do and why?

This adds a merged_by_id or the latest_closed_by_id to a MergeRequestMetrics record when importing pull requests from bitbucket cloud. It assigns either the mapped BitBucket User if one can be found, or the merge request author.

Updating these DB columns ensures the MRs have a user assigned as the Merged by or Closed by user.

MR acceptance checklist

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Screenshots or screen recordings

rec1

Related to #468852 (closed)

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

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