Add container registry online GC dashboard

Context

The new container registry backed by a metadata database has been deployed to pre-prod (gitlab-org&5392 (closed)) and we're now working on adding as many metrics as possible to the registry Grafana dashboards as part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12795.

Changes

This MR adds a new dashboard for online garbage collection (link - only pre-prod has data for this dashboard):