Add Prometheus support for app monitoring

[lostintangent]

We should show how to install Prometheus into the k8s cluster via Helm, and illustrate how to track key app/server metrics with it.

See this article for an example of how this is done: https://askmike.org/articles/monitoring-node-apps-with-prometheus

[nicolehaugen]

Note that Prometheus is an open source option to App Insights. Jonathan added this as a potential future addition to the sample - this isn't our immediate focus, so this won't be assigned to as sprint work for now.