Add remaining DB row count metrics and update Grafana dashboards and alerts

Context

This is a followup from Adjust online GC row count metrics (#1250 - closed).

Task

Once the changes from #1250 (closed) are validated, we'll need to:

1. Add a new row count metric registry_database_rows{query_name="gc_manifest_review_queue"} to collect the number of tasks in the manifest review queue (similar to the one added for blobs in !2432 (merged));
2. Add new row count metrics registry_database_rows{query_name="gc_[blob|manifest]_review_queue_overdue"} to collect the number of tasks who's review is overdue (WHERE review_after < NOW());
3. Update online GC grafana dashboard by replacing all instance of gitlab_database_rows with registry_database_rows (the metric labels are the same, so this should be all that's needed) here
4. Do the same as above but for the alerts here.
5. Validate that the corresponding graphs in the Grafana dashboard are rendered correctly and values match what we see on a registry DB production replica. Side note: While at it and if it's simple, it seems that the formatting of this dashboard is broken.