Prometheus.erl support.

Hello!

I want to add [prometheus.erl](https://github.com/deadtrickster/prometheus.erl) support.

Metrics:
- counter - non-negative number. Always can only go up (or reset);
- gauge - number. Can be incremented/decremented or set to arbitrary value;
- summary - right now just samples count and sum;
- histogram - samples count/sum + per bucket count.

How this could be mapped to erlang-metrics metrics?

Thank you

Start by reviewing the erlang-metrics interface and the prometheus.erl project named in the issue. Compare how counters, gauges, summaries, and histograms map to the available metrics model. Done means the mapping is agreed and Prometheus support covers the listed metric types.

Written by the indexing model from the issue text.