feature request: record metrics and export to prometheus

[gdubya]

Apologies if this functionality already exists and I've missed it.

It would be great if the program could record metrics and then make them available to Prometheus (e.g. using the prometheus_exporter crate).

(I appreciate that I can add the functionality myself and send a PR; this task is a placeholder for discussion, in case anyone else works on it in the meantime ;))