Check gstg registry DB access and configuration for monitoring tools

We use several tools for generating DB metrics. Those need access to the database and need to be configured to do the right queries for the registry DB (some of those tools expect tables found on the gitlab DB, which are not present in the registry DB).

Symptoms are connection failures like this for postgres_exporter in the DB logs:

role ""postgres_exporter"" does not exist

• add roles for postgres_exporter, gitlab-monitor, gitlab-consul
• add missing extensions, functions
• replication lag metrics maybe need adjustment for PG12 (some fields are named differently?)
• check pgbouncer logs / postgres logs for any other failing queries