prometheus: Add CollectorRegistry.unregister functions

[last-genius]

Sources of metrics can sometimes disappear at runtime (a VM being destroyed, disk being unplugged), and rather than continuing to present the last value, it is useful to remove them altogether.

Other Prometheus client libraries do implement similar functions as well, see https://docs.rs/prometheus/latest/prometheus/struct.Registry.html#method.unregister for example.

[avsm]

This looks good to me; I'll add it into my release queue unless anyone has any further thoughts (@talex5?)

[talex5]

unregister_labels does mean you can now use labels to get a metric, then do unregister_labels and end up with a reference to a metric that will be ignored. Maybe there should be a warning about that in the documentation?

[avsm]

On that note, the prometheus spec recommends:

CollectorRegistry SHOULD offer register()/unregister() functions, and a Collector SHOULD be allowed to be registered to multiple CollectorRegistrys.
...
Metrics with labels SHOULD support a remove() method with the same signature as labels() that will remove a Child from the metric no longer exporting it, and a clear() method that removes all Children from the metric. These invalidate caching of Children.