feat: expose impact metrics (#10151)

src/lib/features/metrics/impact/impact-register.ts

@@ -0,0 +1,5 @@
+import { Registry } from 'prom-client';
+
+// Deliberately keep it separate from the global instance metrics register
+// It is a singleton that everyone participating in the impact metrics can import
+export const impactRegister = new Registry();

src/lib/routes/backstage.ts

@@ -2,6 +2,7 @@ import { writeHeapSnapshot } from 'v8';
 import { tmpdir } from 'os';
 import { join } from 'path';
 import { register as prometheusRegister } from 'prom-client';
+import { impactRegister } from '../features/metrics/impact/impact-register.js';
 import Controller from './controller.js';
 import type { IUnleashConfig } from '../types/option.js';
 import type { IFlagResolver } from '../types/index.js';
@@ -41,6 +42,16 @@ class BackstageController extends Controller {
 
                 res.end(metricsOutput);
             });
+
+            if (this.flagResolver.isEnabled('impactMetrics')) {
+                this.get('/impact/metrics', async (req, res) => {
+                    res.set('Content-Type', impactRegister.contentType);
+
+                    const metricsOutput = await impactRegister.metrics();
+
+                    res.end(metricsOutput);
+                });
+            }
         }
 
         if (config.server.enableHeapSnapshotEnpoint) {

src/lib/types/experimental.ts

@@ -60,6 +60,7 @@ export type IFlagKey =
     | 'lastSeenBulkQuery'
     | 'lifecycleMetrics'
     | 'customMetrics'
+    | 'impactMetrics'
     | 'createFlagDialogCache'
     | 'improvedJsonDiff'
     | 'changeRequestApproverEmails';