Listen on separate port

[dobesv]

To avoid exposing the metrics to anyone accessing the site, it would be nice to be able to instrument the server without adding the /metrics route to it.

[dobesv]

That is, I'd like to create an http listener on port 3001 which only responds to /metrics with metrics, but isn't instrumented. And I'd like to collect metrics from my "main" express server.

[hsingh6764]

#54

[roylines]

Happy to merge, just waiting on my couple of "asks" in #54