Create Cross Cluster Heartbeat Metric

[vjkoskela]

We should create a metric for the inter-node heartbeat interval measured by Akka* Exporting it to AMP via the existing OSS AMP client is sufficient since I am working on exporting all those metrics to Vortex2* I believe these are reasonable places to start:   

- https://doc.akka.io/docs/akka/2.5/remoting.html#watching-remote-actors
- https://doc.akka.io/docs/akka/2.5/general/configuration.html (see "failure-detector")
- https://github.com/akka/akka/blob/v2.5.30/akka-remote/src/main/scala/akka/remote/PhiAccrualFailureDetector.scala#L147

The solution that I would recommend is subclassing/encapsulating the standard detector and injecting our class with MetricsFactory to record the heartbeat metrics (by destination node).

[BrandonArp]

Let me throw out the idea that source and destination are tags on the metric. This could give a good idea of connectivity/latency and allow you to slice it pretty well for visualization.