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

well, in our tests, the sigar collector https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala#L695
has never worked, because the constructor
https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala#L703
would inject ```ref=Success(Sigar)```, not the the ```ref=Sigar```, so the following check
https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala#L767
https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala#L728
would result in polite "info" message
https://github.com/akka/akka/blob/master/akka-cluster/src/main/scala/akka/cluster/ClusterMetricsCollector.scala#L786

and since there are no specific specs involving Sigar presence during ```test:test*```
that was left unnoticed. 

so the motivation for this PR:
1) make sigar usage type-safe
2) introduce easy sigar provisioning ##16355 so ```test:test*``` can involve sigar