What
Add a Prometheus exporter to App Metrics service in the Metrics APB

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

How
Expose a 'metrics' endpoint that automatically gets polled by Prometheus.
The Golang client may help with this https://github.com/prometheus/client_golang

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