Add Postgres dashboards with alerting

Created by: daxmc99

Continuation of https://github.com/sourcegraph/sourcegraph/issues/14451

We have access to Postgres metrics in a Sourcegraph installation but we currently have no alerts around this.

We should have 2 seperate dashboards for each Postgres database. The LSIF database likely has different concerns than the general database.

This complements our existing, cloud-only PgHero deployment

cc @efritz , cc @tsenart