Update outdated `merge_requet_metrics` records with latest `events` data

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15642 caches events information into merge_request_metrics using a background migration. While the migration doesn't fully run we make a conditional usage of MergeRequest#metrics, serializing it only if exists, otherwise making the old query on events.

After the migration fully runs, we're able to remove the condition and directly serialize MergeRequest#metrics.