Export a method to get prometheus metrics without mounting to /metrics

[raymondfeng]

I would like to use appmetrics with https://github.com/strongloop/loopback-next/pull/3339. The issue is that we want to control the REST endpoints. It would be nice to have a method from this module to get prometheus metrics without mounting to /metrics.

[sam-github]

@raymondfeng do you want the numeric values, or do you want a function(req, res) ... that you can mount on an express path explicitly?

[raymondfeng]

@sam-github I would prefer to get numeric values or the textual metrics in prometheus formats as we expose more metrics beyond appmetrics.

[sam-github]

That makes sense, and should be not too hard, I will try to find time in the next weeks.