Add 'done' metric log for message tracing

packages/node_modules/@node-red/runtime/lib/nodes/Node.js

@@ -124,6 +124,7 @@ Node.prototype.context = function() {
  * @param  {error} error (optional) an error hit whilst handling the message
  */
 Node.prototype._complete = function(msg,error) {
+    this.metric("done",msg);
     if (error) {
         // For now, delegate this to this.error
         // But at some point, the timeout handling will need to know about