Importers don't use Merge Request metrics

With the addition of the merge_request_metrics table, we now see "Merged by X in SHA Y` messages. However, all our importers (e.g. GitHub, Bitbucket, etc.) don't use this, so imported merge requests like something like:

We should modify the importers to pull in this data. If this data doesn't exist, we should leave it out of the widget.

/cc: @victorwu