Provide metrics::add_extra_producer() to enable external metrics

[bobrik]

This is a more forward and backward compatible solution than #65.

[dqminh]

This still depends on an implicit assumption that foundations and the additional producers produce only in text format which is very stable, so don't think it will change. If either uses different encoding for whatever reasons, then we will produce broken format.

To make this goaway probably still require the same prometheus_client as seen with #65