Provide a list of metric families this Collector is expected to return.
These should exclude the samples. This is used by the registry to
detect collisions and duplicate registrations.
Usually custom collectors do not have to implement Describable. If
Describable is not implemented and the CollectorRegistry was created
with auto describe enabled (which is the case for the default registry)
then collect will be called at registration time instead of
describe. If this could cause problems, either implement a proper
describe, or if that's not practical have describe return an empty