Add dashboard for registry metrics on the Rails side

Related to Container Registry: Port row count queries from... (gitlab-org/gitlab#382513 - closed). This adds a new dashboard to display some database row count metrics that are used to monitor the status of container registry tag cleanup policies and async repository deletions, which all happen on the Rails side.

https://dashboards.gitlab.net/dashboard/snapshot/RZcwDuyEl6Z3w1PwTQBaWkiBoybfhBWJ?orgId=1