feat(metrics): Expose toggle updates to prometheus

closes #323

docs/api/internal-backstage-api.md

@@ -6,3 +6,14 @@ Unleash uses prometheus internally to collect metrics. These are
 available on the given url if the `serverMetrics` option is enabled (default=true).
 
 [Read more about Prometheus](https://prometheus.io/) 
+
+## Annotations
+
+Unleash will automatically count all updates for all toggles under the metric name `feature_toggle_update_total`, and the toggle name is will be set as a label value. This information can be used to create annotations in grafana for everytime a feature toggle is changed. 
+
+You can use this query in grafana to achive this:
+
+```
+delta(feature_toggle_update_total{toggle="Demo"}[1m]) != bool 0?
+```
+

lib/metrics.js

@@ -1,8 +1,14 @@
 'use strict';
 
 const events = require('./events');
+const {
+    FEATURE_CREATED,
+    FEATURE_UPDATED,
+    FEATURE_ARCHIVED,
+    FEATURE_REVIVED,
+} = require('./event-type');
 
-exports.startMonitoring = (enable, eventBus) => {
+exports.startMonitoring = (enable, eventBus, eventStore) => {
     if (!enable) {
         return;
     }
@@ -19,8 +25,26 @@ exports.startMonitoring = (enable, eventBus) => {
         labelNames: ['path', 'method', 'status'],
         percentiles: [0.1, 0.5, 0.9, 0.99],
     });
+    const featureToggleUpdateTotal = new client.Counter({
+        name: 'feature_toggle_update_total',
+        help: 'Number of times a toggle has  been updated',
+        labelNames: ['toggle'],
+    });
 
     eventBus.on(events.REQUEST_TIME, ({ path, method, time, statusCode }) => {
         requestDuration.labels(path, method, statusCode).observe(time);
     });
+
+    eventStore.on(FEATURE_CREATED, ({ data }) => {
+        featureToggleUpdateTotal.labels(data.name).inc();
+    });
+    eventStore.on(FEATURE_UPDATED, ({ data }) => {
+        featureToggleUpdateTotal.labels(data.name).inc();
+    });
+    eventStore.on(FEATURE_ARCHIVED, ({ data }) => {
+        featureToggleUpdateTotal.labels(data.name).inc();
+    });
+    eventStore.on(FEATURE_REVIVED, ({ data }) => {
+        featureToggleUpdateTotal.labels(data.name).inc();
+    });
 };

lib/metrics.test.js

@@ -3,12 +3,17 @@
 const { test } = require('ava');
 const { EventEmitter } = require('events');
 const eventBus = new EventEmitter();
+const eventStore = new EventEmitter();
 const { REQUEST_TIME } = require('./events');
+const { FEATURE_UPDATED } = require('./event-type');
 const { startMonitoring } = require('./metrics');
 const { register: prometheusRegister } = require('prom-client');
 
+test.before(() => {
+    startMonitoring(true, eventBus, eventStore);
+});
+
 test('should collect metrics for requests', t => {
-    startMonitoring(true, eventBus);
     eventBus.emit(REQUEST_TIME, {
         path: 'somePath',
         method: 'GET',
@@ -22,3 +27,12 @@ test('should collect metrics for requests', t => {
         /http_request_duration_milliseconds{quantile="0\.99",path="somePath",method="GET",status="200"} 1337/
     );
 });
+
+test('should collect metrics for updated toggles', t => {
+    eventStore.emit(FEATURE_UPDATED, {
+        data: { name: 'TestToggle' },
+    });
+
+    const metrics = prometheusRegister.metrics();
+    t.regex(metrics, /feature_toggle_update_total{toggle="TestToggle"} 1/);
+});

lib/server-impl.js

@@ -26,7 +26,7 @@ function createApp(options) {
     );
 
     const app = getApp(config);
-    startMonitoring(options.serverMetrics, eventBus);
+    startMonitoring(options.serverMetrics, eventBus, stores.eventStore);
 
     const server = app.listen({ port: options.port, host: options.host }, () =>
         logger.info(`Unleash started on port ${server.address().port}`)