Huge amount of RowExclusiveLocks alert

@pcarranza @yorickpeterse @ahanselka there is no runbook and proper description for this alert. Feel free to add proper texts.

It is alert from the issue - https://gitlab.com/gitlab-com/infrastructure/issues/1135

alerts/gitlab-db-locks.rules

+## DB lock alerts
+ALERT PostgresTooManyRowExclusiveLocks
+  IF sum(pg_locks_count{mode="rowexclusivelock"}) > 100
+  FOR 30m
+  LABELS {severity="critical"}
+  ANNOTATIONS {
+    title="Too many row exclusive locks: {{ $value }}",
+    description="Huge amount of row exclusive locks can result in GitLab.com inavailability. Check http://performance.gitlab.net/dashboard/db/postgres-stats.",
+  }

[yorickpeterse-staging]

I would use:

Huge amounts of ROW EXCLUSIVE locks (used by SQL UPDATEs for example) can result in GitLab.com being unresponsive. See http://performance.gitlab.net/dashboard/db/postgres-stats for more information.