add method which allows adding custom metrics

README.md

@@ -174,6 +174,30 @@ server.listen(port, (err) => {
 });
 ```
 
+## prometheus.addMetric(metricInfo, onDataEvent)
+
+this method allow you to add custom monitoring for metrics.
+the first parameter should have `name`, which represent the name of the appmetrics monitoring event. `data`, the dynamic data which will be saved on each event, and a `toString` which will be printed as part of the `metrics` route.
+the last function is the listener for the monitoring and it will `this` as the metric info, which allow to save the data differently on each metric monitoring event.
+
+```js
+  require('appmetrics-prometheus')
+    .attach()
+    .addMetric({
+      name: 'socketio',
+      data: null,
+      toString() {
+        if(this.data) {
+          return `socketio_last_event ${JSON.stringify(this.data)}`;
+        }
+
+        return '';
+      }
+    }, function(data) {
+      this.data = data;
+    });
+```
+
 ## Performance overhead
 
 Our testing has shown that the performance overhead in terms of processing is minimal, adding less than 0.5 % to the CPU usage of your application.

lib/appmetrics-prometheus.js

@@ -35,20 +35,21 @@ var save = {
   http: {},
   https: {},
 };
-
+var extendedMetrics = {};
+var globalOptions = {};
 
 exports.attach = function(options) {
   if (save.http.Server) {
     // Already attached.
     return exports;
   }
   // Protect our options from modification.
-  options = util._extend({}, options);
+  globalOptions = util._extend({}, options);
 
   // if the user hasn't supplied appmetrics, require
   // here so we get http probe data
-  if (!options.appmetrics) {
-    options.appmetrics = require('appmetrics');
+  if (!globalOptions.appmetrics) {
+    globalOptions.appmetrics = require('appmetrics');
   }
 
   patch(save.http, require('http'));
@@ -59,21 +60,50 @@ exports.attach = function(options) {
     save.Server = http.Server;
     http.Server = function() {
       const server = save.Server.apply(this, arguments);
-      options.server = server;
-      monitor(options);
+      globalOptions.server = server;
+      monitor(globalOptions);
       return server;
     };
     save.createServer = http.createServer;
     http.createServer = function() {
       const server = save.createServer.apply(this, arguments);
-      options.server = server;
-      monitor(options);
+      globalOptions.server = server;
+      monitor(globalOptions);
       return server;
     };
   }
   return exports;
 };
 
+/**
+ * this method allow you to add customized metrics that will be visible through prometheus
+ * metrics endpoint
+ *
+ * @param {Object}   metricInfo
+ * @param {string}   metricInfo.name      the listener monitored metric
+ * @param {any}      metricInfo.data      the changing data of the metric
+ * @param {function} metricInfo.toString  the string representation of this object. will be showed as new line in the metrics endpoint
+ * @param {function} onMetricData         function that represent what to do when receiving metric data,
+ *                                        usually will set the data in order to print the data on metrics endpoint.
+ *
+ * @returns exports                       the exports object for chaining
+ */
+exports.addMetric = function(metricInfo, onMetricData) {
+  var appmetrics = globalOptions.appmetrics || require('appmetrics');
+  var monitoring = appmetrics.monitor();
+
+  if (typeof onMetricData !== 'function') {
+    onMetricData = function(data) {
+      extendedMetrics[metricInfo.name].data = data;
+    };
+  }
+
+  extendedMetrics[metricInfo.name] = metricInfo;
+  monitoring.on(metricInfo.name, onMetricData.bind(extendedMetrics[metricInfo.name]));
+
+  return exports;
+};
+
 // Start monitoring process and subscribe to the data.
 // Don't export monitor
 var monitor = function(options) {
@@ -131,6 +161,11 @@ var monitor = function(options) {
       res.write(httpRequestTotal);
       res.write(httpRequestDuration);
     }
+
+    for (var metric in extendedMetrics) {
+      res.write(extendedMetrics[metric].toString() + '\n');
+    }
+
     res.end();
   }
 
@@ -292,3 +327,4 @@ var monitor = function(options) {
   });
   return server;
 };
+