Add prometheus compatible feature metrics endpoint

lib/metrics.js

@@ -8,7 +8,12 @@ const {
     FEATURE_REVIVED,
 } = require('./event-type');
 
-exports.startMonitoring = (enable, eventBus, eventStore) => {
+exports.startMonitoring = (
+    enable,
+    eventBus,
+    eventStore,
+    clientMetricsStore
+) => {
     if (!enable) {
         return;
     }
@@ -30,6 +35,11 @@ exports.startMonitoring = (enable, eventBus, eventStore) => {
         help: 'Number of times a toggle has  been updated',
         labelNames: ['toggle'],
     });
+    const featureToggleUsageTotal = new client.Counter({
+        name: 'feature_toggle_usage_total',
+        help: 'Number of times a feature toggle has been used',
+        labelNames: ['toggle', 'active'],
+    });
 
     eventBus.on(events.REQUEST_TIME, ({ path, method, time, statusCode }) => {
         requestDuration.labels(path, method, statusCode).observe(time);
@@ -47,4 +57,11 @@ exports.startMonitoring = (enable, eventBus, eventStore) => {
     eventStore.on(FEATURE_REVIVED, ({ data }) => {
         featureToggleUpdateTotal.labels(data.name).inc();
     });
+
+    clientMetricsStore.on('metrics', m => {
+        for (const [feature, { yes, no }] of Object.entries(m.bucket.toggles)) {
+            featureToggleUsageTotal.labels(feature, true).inc(yes);
+            featureToggleUsageTotal.labels(feature, false).inc(no);
+        }
+    });
 };

lib/metrics.test.js

@@ -4,13 +4,14 @@ const { test } = require('ava');
 const { EventEmitter } = require('events');
 const eventBus = new EventEmitter();
 const eventStore = new EventEmitter();
+const clientMetricsStore = 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);
+    startMonitoring(true, eventBus, eventStore, clientMetricsStore);
 });
 
 test('should collect metrics for requests', t => {
@@ -36,3 +37,22 @@ test('should collect metrics for updated toggles', t => {
     const metrics = prometheusRegister.metrics();
     t.regex(metrics, /feature_toggle_update_total{toggle="TestToggle"} 1/);
 });
+
+test('should collect metrics for client metric reports', t => {
+    clientMetricsStore.emit('metrics', {
+        bucket: {
+            toggles: {
+                TestToggle: {
+                    yes: 10,
+                    no: 5,
+                },
+            },
+        },
+    });
+
+    const metrics = prometheusRegister.metrics();
+    t.regex(
+        metrics,
+        /feature_toggle_usage_total{toggle="TestToggle",active="true"} 10\nfeature_toggle_usage_total{toggle="TestToggle",active="false"} 5/
+    );
+});

lib/server-impl.js

@@ -26,7 +26,12 @@ function createApp(options) {
     );
 
     const app = getApp(config);
-    startMonitoring(options.serverMetrics, eventBus, stores.eventStore);
+    startMonitoring(
+        options.serverMetrics,
+        eventBus,
+        stores.eventStore,
+        stores.clientMetricsStore
+    );
 
     const server = app.listen({ port: options.port, host: options.host }, () =>
         logger.info(`Unleash started on port ${server.address().port}`)