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

@patriknw have to revert ```()```:
```
[warn] /home/work/git/scala_akka/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala:722: non-nullary method overrides nullary method
[warn]   override def metrics(): Set[Metric] = {
[warn]                ^

```
enough warrings already? :-)