Add autodiscovery for pg_exporter

[idoqo]

The postgres exporter for Prometheus already supports database autodiscovery, making it possible to automatically start monitoring a new database service as soon as they are added.

It will be great if we can add this support for spinup

[idoqo]

Looks like pg_exporter currently only supports database-level auto-discovery (for the same server). We may have to go with running multiple pg_exporter instances (for each database server).