Remove prometheus depenedency from backend-common

.changeset/rare-lemons-hug.md

@@ -0,0 +1,83 @@
+---
+'@backstage/backend-common': minor
+---
+
+Removes the Prometheus integration from `backend-common`.
+
+Rational behind this change is to keep the metrics integration of Backstage
+generic. Instead of directly relying on Prometheus, Backstage will expose
+metrics in a generic way. Integrators can then export the metrics in their
+desired format. For example using Prometheus.
+
+To keep the existing behavior, you need to integrate Prometheus in your
+backend:
+
+First, add a dependency on `express-prom-bundle` and `prom-client` to your backend.
+
+```diff
+// packages/backend/package.json
+  "dependencies": {
++   "express-prom-bundle": "^6.1.0",
++   "prom-client": "^12.0.0",
+```
+
+Then, add a handler for metrics and a simple instrumentation for the endpoints.
+
+```typescript
+// packages/backend/src/metrics.ts
+import { useHotCleanup } from '@backstage/backend-common';
+import { RequestHandler } from 'express';
+import promBundle from 'express-prom-bundle';
+import prom from 'prom-client';
+import * as url from 'url';
+
+const rootRegEx = new RegExp('^/([^/]*)/.*');
+const apiRegEx = new RegExp('^/api/([^/]*)/.*');
+
+export function normalizePath(req: any): string {
+  const path = url.parse(req.originalUrl || req.url).pathname || '/';
+
+  // Capture /api/ and the plugin name
+  if (apiRegEx.test(path)) {
+    return path.replace(apiRegEx, '/api/$1');
+  }
+
+  // Only the first path segment at root level
+  return path.replace(rootRegEx, '/$1');
+}
+
+/**
+ * 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
+  useHotCleanup(module, () => 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,
+    promClient: { collectDefaultMetrics: {} },
+  });
+}
+```
+
+Last, extend your router configuration with the `metricsHandler`:
+
+```diff
++import { metricsHandler } from './metrics';
+
+...
+
+  const service = createServiceBuilder(module)
+    .loadConfig(config)
+    .addRouter('', await healthcheck(healthcheckEnv))
++   .addRouter('', metricsHandler())
+    .addRouter('/api', apiRouter);
+```
+
+Your Prometheus metrics will be available at the `/metrics` endpoint.