feat(metrics): Add third-party dep go-metrics

github.com/rcrowley/go-metrics commit 880a57cfbce579be3970f997eb8f589ba0b03211
feat(metrics): Add go-metrics factory code

* Add ability to push metrics to graphite
* Wrap go-metrics in a factory to return NOP timers when a graphite host
  is not provided
feat(metrics): Report timer.ae.handle metric
DO NOT MERGE - push this to goraft/raft!

* Time completion of commands by name
* Time peer heartbeats by name