expose prometheus metrics as prometheus collector instances

[AmitKumarDas]

ProblemStatement: When I want to use metac as a library, I want to feed metac's prometheus metrics to the metrics server that my application is supposed to expose. This is turn also implies metac to optionally bring up a metrics server. Metac should instead provide ways to use its metrics collections by its consumers.

[grzesuav]

duplicate of #113