Move all non-postgres monitoring from postgres_exporter to gitlab-monitor

TL;DR: We should keep only postgres related monitoring in the postgres_exporter and move system-wide Gitlab state monitoring to gitlab-monitor.

There have been several queries added to queries.yml for postgres_exporter that are actually monitoring global gitlab state. We have a more appropriate tool for such monitoring, gitlab-monitor. These queries should all be moved there. This will avoid having duplicate metrics for each database, it can also be configured with more appropriate prometheus scrape config, and most importantly it's Ruby code so it can access Redis or other state and export richer information. It also can cache query results which means we could run more expensive queries more reasonably.