Monitor database bloat

This adds database bloat monitoring. It returns metrics useful to understand bloat levels in the database on a per-index and per-table granularity. Note the queries only perform an estimation, this is not an exact measure (with no upper bound on the error).

Goes together with: https://ops.gitlab.net/gitlab-cookbooks/chef-repo/merge_requests/837

Issue: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6178