feat(backend): add Prometheus monitoring and metrics

backend/metrics/http-server.js

@@ -0,0 +1,53 @@
+import http from "http";
+
+let Logger;
+
+export default class HttpServer {
+  constructor(host, port, callback, logger) {
+    this.host = host;
+    this.port = port;
+    this.requestCallback = callback;
+    Logger = logger;
+    this.server = null;
+  }
+
+  start () {
+    this.server = http.createServer(this.requestCallback)
+      .on('error', this.handleError.bind(this))
+      .on('clientError', this.handleError.bind(this));
+  }
+
+  close (callback) {
+    if (this.server) {
+      return this.server.close(callback);
+    }
+  }
+
+  handleError (error) {
+    if (error.code === 'EADDRINUSE') {
+      Logger.warn({
+        host: this.host, port: this.port,
+      }, "EADDRINUSE, won't spawn HTTP server");
+      if (this.server) {
+        this.server.close();
+      }
+    } else if (error.code === 'ECONNRESET') {
+      Logger.warn({ errorMessage: error.message }, "HTTPServer: ECONNRESET ");
+    } else {
+      Logger.error(error, "Returned error");
+    }
+  }
+
+  getServerObject() {
+    return this.server;
+  }
+
+  listen(callback) {
+    Logger.info(`HTTPServer is listening: ${this.host}:${this.port}`);
+    if (this.server) {
+      this.server.listen(this.port, this.host, callback);
+    } else {
+      Logger.error("Server not started, call start() before listen()");
+    }
+  }
+}

backend/metrics/index.js

@@ -0,0 +1,40 @@
+import { Counter, Histogram } from 'prom-client';
+
+const metrics = {
+  apiCallsTotal: new Counter({
+    name: 'feedback_app_api_calls_total',
+    help: 'Total number of API calls made',
+    labelNames: ['api'],
+  }),
+  apiCallErrorsTotal: new Counter({
+    name: 'feedback_app_api_call_errors_total',
+    help: 'Total number of API call errors',
+    labelNames: ['api'],
+  }),
+  webhookEventsTotal: new Counter({
+    name: 'feedback_app_webhook_events_total',
+    help: 'Total number of webhook events received',
+    labelNames: ['event_type'],
+  }),
+  webhookErrorsTotal: new Counter({
+    name: 'feedback_app_webhook_errors_total',
+    help: 'Total number of errors processing webhooks',
+  }),
+  feedbackRegistrationsTotal: new Counter({
+    name: 'feedback_app_registrations_total',
+    help: 'Total number of feedbacks registered',
+    labelNames: ['source'], // 'web', 'mobile', 'skipped', 'unknown'
+  }),
+  feedbackFailuresTotal: new Counter({
+    name: 'feedback_app_failures_total',
+    help: 'Total number of feedback registration failures',
+    labelNames: ['reason'], // 'missing_session_or_user', 'already_submitted', 'internal_error'
+  }),
+  feedbackRatingsHistogram: new Histogram({
+    name: 'feedback_app_ratings_histogram',
+    help: 'Histogram of feedback ratings',
+    buckets: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
+  }),
+};
+
+export default metrics;

backend/metrics/prometheus-agent.js

@@ -0,0 +1,134 @@
+import {
+  register,
+  collectDefaultMetrics,
+} from 'prom-client';
+import HTTPServer from './http-server.js';
+
+let Logger;
+
+export default class PrometheusScrapeAgent {
+  constructor (host, port, options, logger) {
+    this.host = host;
+    this.port = port;
+    this.metrics = {};
+    this.started = false;
+    Logger = logger;
+
+    this.path = options.path || '/metrics';
+    this.collectDefaultMetrics = options.collectDefaultMetrics || false;
+    this.metricsPrefix = options.prefix || '';
+    this.collectionTimeout = options.collectionTimeout || 10000;
+  }
+
+  getMetric (name) {
+    return this.metrics[name];
+  }
+
+  async collect (response) {
+    try {
+      response.writeHead(200, { 'Content-Type': register.contentType });
+      const content = await register.metrics();
+      response.end(content);
+    } catch (error) {
+      response.writeHead(500)
+      response.end(error.message);
+      Logger.error(error, 'Prometheus: error collecting metrics');
+    }
+  }
+
+  getMetricsHandler (request, response) {
+    switch (request.method) {
+      case 'GET':
+        if (request.url === this.path) return this.collect(response);
+        response.writeHead(404).end();
+        break;
+      default:
+        response.writeHead(501)
+        response.end();
+        break;
+    }
+  }
+
+  start (requestHandler = this.getMetricsHandler.bind(this)) {
+    if (this.collectDefaultMetrics) collectDefaultMetrics({
+      prefix: this.metricsPrefix,
+      timeout: this.collectionTimeout,
+    });
+
+    this.metricsServer = new HTTPServer(this.host, this.port, requestHandler, Logger);
+    this.metricsServer.start();
+    this.metricsServer.listen();
+    this.started = true;
+  }
+
+  injectMetrics (metricsDictionary) {
+    this.metrics = { ...this.metrics, ...metricsDictionary }
+  }
+
+  increment (metricName, labelsObject) {
+    if (!this.started) return;
+
+    const metric = this.metrics[metricName];
+    if (metric) {
+      metric.inc(labelsObject)
+    }
+  }
+
+  incrementBy (metricName, value, labelsObject) {
+    if (!this.started) return;
+
+    const metric = this.metrics[metricName];
+
+    if (metric) {
+      metric.inc(labelsObject, value)
+    }
+  }
+
+  decrement (metricName, labelsObject) {
+    if (!this.started) return;
+
+    const metric = this.metrics[metricName];
+    if (metric) {
+      metric.dec(labelsObject)
+    }
+  }
+
+  set (metricName, value, labelsObject = {}) {
+    if (!this.started) return;
+
+    const metric = this.metrics[metricName];
+    if (metric) {
+      metric.set(labelsObject, value)
+    }
+  }
+
+  setCollectorWithGenerator (metricName, generator) {
+    const metric = this.getMetric(metricName);
+    if (metric) {
+      metric.collect = () => {
+        metric.set(generator());
+      };
+    }
+  }
+
+  setCollector (metricName, collector) {
+    const metric = this.getMetric(metricName);
+
+    if (metric) {
+      metric.collect = collector.bind(metric);
+    }
+  }
+
+  observe(metricName, value, labelsObject = {}) {
+    if (!this.started) return null;
+
+    const metric = this.metrics[metricName];
+
+    if (metric) {
+      metric.observe(labelsObject, value);
+      return metric;
+    }
+
+    return null;
+  }
+}

backend/package.json

@@ -7,6 +7,7 @@
     "body-parser": "^1.19.0",
     "express": "^4.19.2",
     "pino": "^9.3.2",
+    "prom-client": "^15.1.3",
     "redis": "^4.7.0"
   },
   "scripts": {