Add query to count registry online GC tasks that are overdue

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14127.

We have added queries to count all rows in the registry online GC review tables (https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/151) and this MR expands on that with two new queries that count only rows whose review_after timestamp is in the past. This allows us to monitor not only how many tasks exist in total but also how many are ready to be processed.

review_after is indexed in both gc_manifest_review_queue and gc_blob_review_queue, so these queries will scale well.

I have a pending followup MR for chef: https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/672

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14304.