Proposal: Prometheus metrics

[chapsuk]

Description

Now metrics collecting in internal metrics.Registry and can be pushed to external influxdb.
I suggest adding support for prometheus pull metrics.

Changes

• add prometheus http server for expose metrics;
• format internal metrics to prometheus format

Types mapping

geth	prometheus	description
counter	gauage	geth counter can be decremented, prometheus counter not
gauage	gauage
gauageFloat64	gauage
metr	counter	expose only count metric, prometheus aggregations or rates
timer	summary	expose counter and percentiles, min/max/mean aggregations on prometheus side
resettingTImer	summary

Implementation

For implement expose metrics i'm not use official prometheus client to avoid duplicate metrics aggregation.
I considered the option with the implementation of the metrics.Registry interface, but due to the different types and approach to aggregating metrics, this option did not look friendly.

#17077

Example of grafana dashboard with geth metrics collected by prometheus:
https://snapshot.raintank.io/dashboard/snapshot/nxZB4UEhyo2zvMZNzBUfb7jlTAnE57fF?orgId=2 -

[chapsuk]

#17077 🎉 🌮