[WIP] Starting with metrics and prometheus

[magmax]

Fixes #1320

WORK IN PROGRESS

It is not fully functional right now. First of all, it seems that my local environment doesn't accomplish all the requirements to run the pre-commit and pre-push hooks, but I wanted to share my current code with you.

The idea is:

1. Plugins have the ability to send metrics, but they do not know anything about prometheus.
2. Prometheus plugin receives the message and process it.

Problems:

• prometheus_client has a singleton REGISTRY.
• it is not possible to register two metrics with same name in the REGISTRY.
• but I didn't want to use prometheus_client.REGISTRY on core.

[abhinavsingh]

@magmax Thanks for the initial PR. I took a deep look and it required a different strategy to work w/ proxy.py. Please check PR #1447 which currently is in working state.