Ensure monitoring parity for Sourcegraph.com Cloud SQL

Created by: daxmc99

We recently began using Cloud SQL (a managed database offering). In order to ensure we do not lose observability, we should ensure we have the same alerts and monitoring for Cloud SQL.

https://sourcegraph.com/-/debug/grafana/d/rNCct7gGk/postgresql-statistics?orgId=1 should still provide some level of statistics or direct the user to the equivalent.

Some basic requirements

- [ ] Alerting on error rates - [ ] Alert on slow queries ~~- [ ] Storage ~~ - [ ] Integrated into Prometheus and our current monitoring infrastructure

Personal nice to have: alert #database channel on slow queries (in case people want to try to optimize them)

Part of https://github.com/sourcegraph/sourcegraph/issues/11496