feat(backend-common): introduce metrics for the backend

Add a /metrics endpoint that exposes metrics about the backend. It provides general runtime metrics (memory, gc, utilization) as well as request metrics.

packages/backend-common/package.json

@@ -37,11 +37,13 @@
     "compression": "^1.7.4",
     "cors": "^2.8.5",
     "express": "^4.17.1",
+    "express-prom-bundle": "^6.1.0",
     "express-promise-router": "^3.0.3",
     "helmet": "^4.0.0",
     "knex": "^0.21.1",
     "lodash": "^4.17.15",
     "morgan": "^1.10.0",
+    "prom-client": "^12.0.0",
     "selfsigned": "^1.10.7",
     "stoppable": "^1.1.0",
     "winston": "^3.2.1"

packages/backend-common/src/service/lib/ServiceBuilderImpl.ts

@@ -37,6 +37,7 @@ import {
   HttpsSettings,
 } from './config';
 import { createHttpServer, createHttpsServer } from './hostFactory';
+import { metricsHandler } from './metrics';
 
 const DEFAULT_PORT = 7000;
 // '' is express default, which listens to all interfaces
@@ -131,6 +132,7 @@ export class ServiceBuilderImpl implements ServiceBuilder {
     }
     app.use(compression());
     app.use(express.json());
+    app.use(metricsHandler());
     app.use(requestLoggingHandler());
     for (const [root, route] of this.routers) {
       app.use(root, route);

packages/backend-common/src/service/lib/metrics.ts

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+import prom from 'prom-client';
+import promBundle from 'express-prom-bundle';
+import { RequestHandler } from 'express';
+
+/**
+ * Adds a /metrics endpoint, register default runtime metrics and instrument the router.
+ */
+export function metricsHandler(): RequestHandler {
+  // We can only initialize the metrics once and have to clean them up between hot reloads
+  prom.register.clear();
+
+  return promBundle({
+    includeMethod: true,
+    includePath: true,
+    // Using includePath alone is problematic, as it will include path labels with high
+    // cardinality (e.g. path params). Instead we would have to template them. However, this
+    // is difficult, as every backend plugin might use different routes. Instead we only take
+    // the first directory of the path, to have at least an idea how each plugin performs:
+    normalizePath: [['^/([^/]*)/.*', '/$1']],
+    promClient: { collectDefaultMetrics: {} },
+  });
+}