Add Prometheus support

[maxifom]

It would be nice to have Prometheus alongside statsd one.

IMHO I think Prometheus is easier for devs and also a more popular choice.

If anyone wants to write metrics directly to prometheus without statsd server, there is a solution: https://github.com/prometheus/statsd_exporter

[jonashaag]

cc @Gufran

[Gufran]

@maxifom how do you imagine the prometheus stats to work? I don't think it is a good idea to push the metrics from bjoern to a prometheus sink. And to implement the hook for an exporter we'll need to build a webserver and an endpoint in bjoern that can expose the metrics.
Just looking for ideas here.