Emit server-added and server-removed stat on ringChanged event

CHANGES.md

@@ -4,6 +4,7 @@ ringpop-node release notes
 Future
 -------
 * Remove tick-cluster.js, the version from ringpop-common should be used [#278](https://github.com/uber/ringpop-node/pull/278)
+* Emit server-added and server-removed stat on ringChanged event [#281](https://github.com/uber/ringpop-node/pull/281)
 
 10.15.0
 -------

lib/on_ring_event.js

@@ -42,6 +42,8 @@ function createChecksumComputedHandler(ringpop) {
 function createRingChangedHandler(ringpop) {
     return function onRingChanged(event) {
         ringpop.stat('increment', 'ring.changed');
+        ringpop.stat('increment', 'ring.server-added', event.added);
+        ringpop.stat('increment', 'ring.server-removed', event.removed);
         ringpop.emit('ringChanged', event);
     };
 }