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

1) config name space
```akka.cluster.metrics``` looks nice, but feels wrong, since there no "vertical" relationship
```akka.cluster-toolbox.metrics``` makes it look and feel "horizontal", as it should.

2) package name space
```akka.cluster.metrics``` is nice by itself, but becomes wrong if you conciser other things in akka-contrib;
but if you introduce ```akka.cluster.tools.*``` then you have created a logical level which separates cluster core from non-core

3) either way, need your final choice