GELF appender can add custom fields

lib/appenders/gelf.js

@@ -47,9 +47,33 @@ function gelfAppender (layout, host, port, hostname, facility) {
   process.on('exit', function() {
     if (client) client.close();
   });
-  
+
+  /**
+   * Add custom fields (start with underscore ) 
+   * - if the first object passed to the logger contains 'GELF' field, add copy the underscore fields to the message
+   * @param loggingEvent
+   * @param msg
+   */
+  function addCustomFields(loggingEvent, msg){
+      var data = loggingEvent.data;
+      if (!Array.isArray(data) || data.length === 0) return;
+      var firstData = data[0];
+      if (!firstData['GELF']) return; // identify with GELF field defined
+      var keys = Object.keys(firstData);
+      for (var i in keys){
+          var key = keys[i];
+          // skip _id field for graylog2, skip keys not starts with UNDERSCORE
+          if (!key.match(/^_/) || key === "_id") continue; 
+          msg[key] = firstData[key];
+      }
+      loggingEvent.data.shift(); // remove the first object
+      //console.log('=== msg ===');
+      //console.log(msg);
+  }
+ 
   function preparePacket(loggingEvent) {
     var msg = {};
+    addCustomFields(loggingEvent, msg);
     msg.full_message = layout(loggingEvent);
     msg.short_message = msg.full_message;