Cluster metrics internal API and cluster-wide transport of metrics data.

* Create Cluster Metrics API
* Create transport of relevant metrics data
Does not include load-balancing routers.

akka-cluster/src/main/scala/akka/cluster/ClusterEvent.scala

@@ -88,6 +88,11 @@ object ClusterEvent {
     if (member.status != Removed) throw new IllegalArgumentException("Expected Removed status, got: " + member)
   }
 
+  /**
+   * Current snapshot of cluster member metrics. Published to subscribers.
+   */
+  case class ClusterMetricsChanged(nodes: Set[NodeMetrics]) extends ClusterDomainEvent
+
   /**
    * Cluster convergence state changed.
    */