UpdateMergeRequestsWorker is slow
https://performance.gitlab.net/dashboard/db/sidekiq-workers?orgId=1&var-worker=UpdateMergeRequestsWorker%23perform&var-database=Production&from=now-7d&to=now (warning: takes a long time to load)
It's not easy to tell why this is slow. We spend a lot of time in SQL, which may be due to the changes in how we store MR diff files and commits, but may also be due to the MR row being locked for updates: https://gitlab.com/gitlab-org/gitlab-ce/issues/35470
The big problem here is that we don't appear to have method call timings on the dashboard for these, so we can't tell. That might be a dashboard problem, or it might be an instrumentation problem. We should fix that, then fix the slowness.