Clean up metrics by label

[DifferentialOrange]

It is possible to unregister a whole collector with registry internal methods. But it is not possible (at least without weird for-cycles and table removes) to unregister a specific metric value.

Let's consider the following case. We have a pool of connectors and several metrics of existing connectors (e.g. status, current pending requests, process successes and errors). If connector is deleted, it is no more needed to store its metrics. it would be a good pattern to use a single collector with different labels for a single metric, but then we wouldn't be able to clean it up if needed.