Remove row count probes, metrics and alerts
As part of Drive the Container Registry migration Phase 2 ... (&7316 - closed) we added multiple query row count probes, Prometheus metrics, and alerts to monitor the migration. As we prepare to remove the migration source code on the Rails side, we must first remove these as some rely on database columns that will disappear (#409873) and therefore would cause false alarms to be triggered.
Most of these changes occurred in #357515 (closed).
Edited by João Pereira