I'm don't think we should do this at the enabled/disable level, since if metrics collection has non-trivial cost we shouldn't do it when UKM is enabled either.

Instead, what we should expose is some method for checking whether or not some metrics would be sampled out, since that will help lower expected collection cost.

Alexei had a good point we should use the same model as UMA where we don't modify behavior so I can close.