Add metrics/debug HTTP endpoint

[judwhite]

The goal is to expose state about the process in a human and machine readable format. The user doesn't necessarily need to use Prometheus, but I'm using it as an example for how to expose metrics.

Prometheus compatible /metrics example from etcd:

• https://github.com/coreos/etcd/blob/master/Documentation/metrics.md

.NET HttpListener:

• https://msdn.microsoft.com/en-us/library/system.net.httplistener(v=vs.110).aspx

Prometheus tutorial:

• https://www.digitalocean.com/community/tutorials/how-to-query-prometheus-on-ubuntu-14-04-part-1
• https://www.digitalocean.com/community/tutorials/how-to-query-prometheus-on-ubuntu-14-04-part-2

General references for API documentation:

• https://openapis.org/
• https://openapis.org/specification
• http://editor.swagger.io