Add table and btree bloat metrics to registry DB dashboard

As a followup from gitlab-org/container-registry#497 (closed), this MR adds table and btree bloat metrics to the registry: Database Detail dashboard so that we can see an overview of all these metrics in a single place

https://dashboards.gitlab.net/dashboard/snapshot/Q0MFFUeeKxznYcZEn55K4MxVMCeu5m6a?orgId=1

Related to gitlab-org/container-registry#497 (closed).