! akka-cluster: metrics collector improvements - resolve #16121

hmm, I'm not sure if this will work, it will mean that just having the jar in the classpath will load the extension, which depends on Cluster extension, which will fail if not ClusterActorRefProvider is configured and so on.

I think we should make cluster metrics opt-in, and the user must define this in application.conf to activate it. That will be a replacement for `enabled = on`