Support allowed_metrics_regex in Prometheus surfacer

[benner]

Describe the feature you'd like and the problem it will solve

cloudwatch and stackdriver surfacers has ability to filter which metrics to emit (allowed_metrics_regex). It would be greatge to have same functionality for Prometheus surfacer. This can be useful in situation where Prometeus is for monitoring cloudprober itself and target metrics is somewhere else.

[manugarg]

I think this is a duplicate of #590. Please feel free to reopen if it is not.

#597 implements a fix for this.