Implement Prometheus backend

[tiedotguy]

This may be a bit difficult to do, as data is generally aggregated by the time it reaches the backend, and dealing with flush intervals and clearing of metrics and things like that will likely involve a bit of refactoring somewhere.

This is distinct from an internal prom statser (#284), although an internal regular statser with prom backend should have similar results.

[dadux]

https://github.com/prometheus/statsd_exporter exist already.

[tiedotguy]

I mean... so does etsy statsd, but here we are with gostatsd :)