Add an optional prometheus endpoint to plebbit rpc

[estebanabaroa]

It's pretty easy to add prometheus metrics using https://www.npmjs.com/package/prom-client

I added some to the plebbit provider https://github.com/plebbit/plebbit-provider/blob/master/src/prometheus.js

then we'll be able to create some grafana dashboards and share them with people, and people will be able to monitor their subs performance and uptime, and even set up alerts for when they are down

it's not high priority but I'm putting it here so we dont forget