What
Add a Prometheus exporter to Postgres in the Metrics APB

Why
So that metrics about postgres can be captured, queried and visualised.
This should be available by default in the Metrics APB if possible

How
See the exporter for Postgres https://github.com/wrouesnel/postgres_exporter

NOTE: As a follow up task, a Grafana Dashboard should be added to the Metrics APB to visualise the new metrics being exposed from Postgres