Capture plugin name under the /api/ prefix for http metrics (#3259)

.changeset/weak-rice-beam.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-common': patch
+---
+
+Capture plugin name under the /api/ prefix for http metrics

packages/backend-common/src/service/lib/metrics.test.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 { normalizePath } from './metrics';
+
+describe('normalizePath', () => {
+  it('should normalize /path to /path', async () => {
+    const path = normalizePath({ url: 'http://server/path' });
+
+    expect(path).toBe('/path');
+  });
+
+  it('should normalize /path/test to /path', async () => {
+    const path = normalizePath({ url: 'http://server/path/test' });
+
+    expect(path).toBe('/path');
+  });
+
+  it('should normalize /api/plugin-name/test to /api/plugin-name', async () => {
+    const path = normalizePath({ url: 'http://server/api/plugin-name/test' });
+
+    expect(path).toBe('/api/plugin-name');
+  });
+});

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

@@ -16,6 +16,22 @@
 import prom from 'prom-client';
 import promBundle from 'express-prom-bundle';
 import { RequestHandler } from 'express';
+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.
@@ -31,7 +47,7 @@ export function metricsHandler(): RequestHandler {
     // 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']],
+    normalizePath,
     promClient: { collectDefaultMetrics: {} },
   });
 }