Rename destroyMetric to unregisterMetric

[fabianfett]

Renamed the PrometheusCollectorRegistry destroy methods to unregister. this is more in line with the Prometheus documentation:

CollectorRegistry SHOULD offer register()/unregister() functions, and a Collector SHOULD be allowed to be registered to multiple CollectorRegistrys.

Further this is more in line with what this function actually does. The metrics aren't move only types, that we can actually destroy in the method.

Also added docc comments.

[FranzBusch]

Are you also planning to rename the make methods?

[fabianfett]

Are you also planning to rename the make methods?

@FranzBusch No, I don't plan this. Reason: We might want to add explicit calls to register and allow create metric creation without a registry attached from the get go. However we could change the internal make implementation to do exactly that.

My goal with this is to get the API in such a shape, that we can be fully compatible with the prometheus requirements without breaking API.

[ktoso]

Okey so both names exist then, that's ok