2018-01-08 Post-deployment migration SchedulePopulateMergeRequestMetricsWithEventsData taking a long time

Slack discussion: https://gitlab.slack.com/archives/C0XM5UU6B/p1515429888000331

Post-deployment migrations are currently on 1.9 hours.

The only output in the deploy nodes production.log is...

Migrating to RemoveAssigneeIdFromIssue (20170523073948)
Migrating to SchedulePopulateMergeRequestMetricsWithEventsData (20171128214150)

Which would suggest that RemoveAssigneeIdFromIssue has completed and SchedulePopulateMergeRequestMetricsWithEventsData is currently running and has been for a long long time.

@oswaldo will be able to offer more info about this migration when he is available.

/cc @marin @smcgivern