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

1) ```strategy``` is left public because since user already paid the price of providing custom strategy, she might as well have access to it for other cases.

2) ```strategy``` is in the extension, not in the supervisor - because, since we depend on class which might be missing, we fail early, during extension init.

3) ```supervisor``` is left public - I had intent (not present in the code) to introduce ```CollectorStart``` and ```CollectorStop``` messages to control collection at run time

WDYT?