Optionally disable Process and Platform collectors

[chas0amx]

Implement options to disable those collectors during usage

Something like this:

REGISTRY.unregister(PROCESS_COLLECTOR)
REGISTRY.unregister(PLATFORM_COLLECTOR)
REGISTRY.unregister(REGISTRY._names_to_collectors['python_gc_duration_seconds_sum'])
REGISTRY.unregister(REGISTRY._names_to_collectors['python_gc_uncollectable_objects_sum'])
REGISTRY.unregister(REGISTRY._names_to_collectors['python_gc_collected_objects_sum'])

Something similar has already discussed in prometheus/client_python#414