Add Prometheus annotations to server pod

We should add the standard annotations to indicate which port to scrape for Prometheus: https://github.com/helm/charts/tree/master/stable/prometheus#scraping-pod-metrics-via-annotations 

metadata:

 annotations:

  prometheus.io/scrape: "true"

  prometheus.io/path: /metrics

  prometheus.io/port: "9091"

This will allow it to be used with the standard helm charts and Prometheus operator, plus a myriad of other tooling, with minimal effort. We do indicate with the exporter to use the helm chart so we should make sure it actually works.