From 4471798adec80a7a0eb27b40673a364790fb9c32 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Pedro=20Alves?= <joaopealves.dev@gmail.com>
Date: Thu, 26 Feb 2026 22:25:58 -0300
Subject: [PATCH] refactor(telemetry): simplify HTTP request metrics and
 integrate automatic instrumentation

---
 .../infra/telemetry/http-request-metrics.ts   | 44 ++++++-------------
 apps/backend/src/infra/telemetry/otel.ts      |  4 ++
 2 files changed, 18 insertions(+), 30 deletions(-)

diff --git a/apps/backend/src/infra/telemetry/http-request-metrics.ts b/apps/backend/src/infra/telemetry/http-request-metrics.ts
index bb7e9cf9..8640d78a 100644
--- a/apps/backend/src/infra/telemetry/http-request-metrics.ts
+++ b/apps/backend/src/infra/telemetry/http-request-metrics.ts
@@ -1,45 +1,29 @@
 import { metrics } from '@opentelemetry/api'
 import type { FastifyInstance } from 'fastify'
 
-function getStatusClass(statusCode: number): 'ok' | 'error' {
-  return statusCode >= 200 && statusCode < 400 ? 'ok' : 'error'
-}
-
+/**
+ * Registers a lightweight request counter on top of the automatic telemetry
+ * provided by @opentelemetry/instrumentation-http and @fastify/otel.
+ *
+ * - http.server.request.duration (histogram) is emitted automatically by
+ *   instrumentation-http following OTel semantic conventions.
+ * - Span status ERROR + exception recording on 5xx is handled automatically
+ *   by instrumentation-http (sets status) and @fastify/otel (recordExceptions).
+ * - This counter adds a simple `http.server.requests` metric useful for
+ *   quick rate-of-requests queries without histogram overhead.
+ */
 export function registerHttpRequestMetrics(app: FastifyInstance) {
   const meter = metrics.getMeter('plotwist-api', '0.1.0')
   const requestCounter = meter.createCounter('http.server.requests', {
-    description: 'HTTP server request count by status',
+    description: 'Total HTTP server requests',
     unit: '1',
   })
-  const requestDuration = meter.createHistogram(
-    'http.server.request.duration',
-    {
-      description: 'HTTP server request duration',
-      unit: 's',
-    }
-  )
-
-  app.addHook('onRequest', (request, _reply, done) => {
-    ;(request as { _startTime?: number })._startTime = Date.now()
-    done()
-  })
 
   app.addHook('onResponse', (request, reply, done) => {
     const statusCode = reply.statusCode
-    const statusClass = getStatusClass(statusCode)
-    const startTime = (request as { _startTime?: number })._startTime
-    const durationSec =
-      typeof startTime === 'number' ? (Date.now() - startTime) / 1000 : 0
-
     requestCounter.add(1, {
-      'http.status_code': statusCode,
-      'http.response.status': statusClass,
-      'http.method': request.method,
-      'http.route': request.routeOptions?.url ?? request.url,
-    })
-    requestDuration.record(durationSec, {
-      'http.response.status': statusClass,
-      'http.method': request.method,
+      'http.response.status_code': statusCode,
+      'http.request.method': request.method,
       'http.route': request.routeOptions?.url ?? request.url,
     })
     done()
diff --git a/apps/backend/src/infra/telemetry/otel.ts b/apps/backend/src/infra/telemetry/otel.ts
index a3264765..ae3bcb4e 100644
--- a/apps/backend/src/infra/telemetry/otel.ts
+++ b/apps/backend/src/infra/telemetry/otel.ts
@@ -3,6 +3,7 @@ import { metrics, trace } from '@opentelemetry/api'
 import { OTLPMetricExporter } from '@opentelemetry/exporter-metrics-otlp-proto'
 import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-proto'
 import { HostMetrics } from '@opentelemetry/host-metrics'
+import { HttpInstrumentation } from '@opentelemetry/instrumentation-http'
 import { resourceFromAttributes } from '@opentelemetry/resources'
 import { PeriodicExportingMetricReader } from '@opentelemetry/sdk-metrics'
 import { NodeSDK } from '@opentelemetry/sdk-node'
@@ -59,6 +60,8 @@ function parseOtlpHeaders(raw: string): Record<string, string> {
 
 const { metricsUrl, tracesUrl, headers } = getOtlpConfig()
 
+const httpInstrumentation = new HttpInstrumentation()
+
 const sdk = new NodeSDK({
   resource: resourceFromAttributes({
     [ATTR_SERVICE_NAME]: 'plotwist-api',
@@ -68,6 +71,7 @@ const sdk = new NodeSDK({
   metricReader: new PeriodicExportingMetricReader({
     exporter: new OTLPMetricExporter({ url: metricsUrl, headers }),
   }),
+  instrumentations: [httpInstrumentation],
 })
 
 logger.info('Starting OTLP exporter')