The change covers two things:
1. Cleanup of Set.a metrics list from per-quantile metrics
for summary.
2. Register summary metric and per-quantile metrics in one take.
This prevents registry corruption when Unregister called in the
middle of metric register process.
* Allow unregistration of metrics
* Provide metrics.WritePrometheusSet to write using a dedicated metric set
* Cleanup unregister metrics patch, add generic WriteProcessMetrics
* Add new type of counter: FloatCounter
* sometimes you need to count things with more precision than uint64
* FloatCounter also usefull if you need setable Gauge w/o callback func
* Fix PR review:
* sync.RWMutex -> sync.Mutex
* more idiomatic add/sub
