fix(custom-metrics): add noop functions

Author: gergelyke

lib/trace.js

@@ -7,7 +7,9 @@ var traceNoop = {
   report: function () {},
   reportError: function () {},
   getTransactionId: function () {},
-  sendMemorySnapshot: function () {}
+  sendMemorySnapshot: function () {},
+  recordMetric: function () {},
+  incrementMetric: function () {}
 }
 
 function Trace (config) {