+ akka-cluster-metrics: new akka module

* new akka module split from akka-cluster
* provides sigar provisioning
* resolve #16121
* see #16354
applied, except did not "place them in the companion object"
since that would make for some ugly sight:
```akka.cluster.metrics.ClusterMetricsCollector$ClusterMetricsMessage```
and ```akka.cluster``` is not doing this to its messages