Add support for Prometheus metrics

[michaeljs1990]

This is a duplicate of Symantec/Dominator#432 that I am moving over to this repo.

Of relevance in that ticket is the following from @rgooch.

I'm not really keen on that. I consider the tricorder/Scotty ecosystem a better solution (it supports more than floats and has a better UI for humans to discover and explore the available metrics for an application), and it follows the vision of more integrations in the metrics space (like integrating with the health-agent for health checking during rollouts). Also, I don't want to add another code dependency and bloat out the system. Note also that the tricorder library creates a /metrics endpoint already.

What might be a good alternative is to add the ability to push metrics from Scotty to Prometheus. The advantage of this approach is that none of the Dominator ecosystem components need to be changed.

Please take a look:
https://github.com/Symantec/scotty
https://docs.google.com/document/d/e/2PACX-1vQPhkHYiLK7aKHLECa9EFtSCBSPK-obgGB8C66d72Kl-ej9NikRKYGsuFj1R9aDTlGiZA7OmXrVw8P3/pub

Is this still the correct path to take for getting prom style metrics out of dominator in a way that can be contributed back upstream? I think some additional conversation may need to happen in the scotty repo mostly around prom scraping scotty instead of scotty pushing to it if that is an option.

[rgooch]

So, an alternative that I've discussed with @cviecco is to modify the tricorder library to also publish Prometheus-compatible metrics. If that can be done cheaply without adding external dependencies, that might be a better approach.