Enable postgres_exporter by default

[paulfantom]

What is missing?

TimescaleDB-single helm chart has an ability to run postgres_exporter. It would be nice to enable this exporter and gather data from the DB by default.

Note that we should change default image to the one built from https://github.com/prometheus-community/postgres_exporter

Why do we need it?

To increase visibility into our stack.

Anything else we need to know?:

[paulfantom]

Additionally it would be nice to ship alerts and dashboards for this particular exporter. Mixin is located in https://github.com/prometheus-community/postgres_exporter/tree/master/postgres_mixin

[paulfantom]

As part of this task we should also ship postgres_exporter mixin (alerts and dashboards) by default.