feat: add prometheus latency histogram for /api/admin

src/metrics/registry.ts

@@ -69,4 +69,22 @@ export function resetMaintenanceMetrics(): void {
   maintenanceDuration.reset();
 }
 
-export { billingDeductDuration, refreshTokenDuration, maintenanceDuration, creditsDuration };
+const adminDuration = new client.Histogram({
+  name: 'admin_duration_seconds',
+  help: 'Latency of /api/admin routes in seconds',
+  labelNames: ['route', 'status_code'],
+  buckets: [0.001, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10],
+});
+
+export function recordAdminDuration(route: string, statusCode: number, durationMs: number): void {
+  adminDuration.observe(
+    { route, status_code: String(statusCode) },
+    durationMs / 1000,
+  );
+}
+
+export function resetAdminMetrics(): void {
+  adminDuration.reset();
+}
+
+export { billingDeductDuration, refreshTokenDuration, maintenanceDuration, creditsDuration, adminDuration };

src/middleware/metricsHistogram.ts

@@ -1,6 +1,7 @@
 import type { Request, Response, NextFunction } from 'express';
 import { performance } from 'node:perf_hooks';
 import {
+  recordAdminDuration,
   recordBillingDeductDuration,
   recordRefreshTokenDuration,
   recordMaintenanceDuration,
@@ -50,3 +51,19 @@ export function maintenanceHistogramMiddleware(
 
   next();
 }
+
+export function adminHistogramMiddleware(
+  req: Request,
+  res: Response,
+  next: NextFunction,
+): void {
+  const start = performance.now();
+  const route = req.baseUrl + (req.route?.path ?? req.path);
+
+  res.on('finish', () => {
+    const durationMs = performance.now() - start;
+    recordAdminDuration(route, res.statusCode, durationMs);
+  });
+
+  next();
+}

src/routes/admin.ts

@@ -2,6 +2,7 @@ import { adminLogMiddleware } from '../middleware/adminLog.js';
 import { Router, type Response } from 'express';
 import { adminAuth } from '../middleware/adminAuth.js';
 import { createAdminIpAllowlist } from '../middleware/ipAllowlist.js';
+import { adminHistogramMiddleware } from '../middleware/metricsHistogram.js';
 import { findUsers } from '../repositories/userRepository.js';
 import { parsePagination, paginatedResponse } from '../lib/pagination.js';
 import { getClientIp } from '../lib/clientIp.js';
@@ -40,6 +41,7 @@ const router = Router();
 router.use(createAdminIpAllowlist());
 router.use(adminAuth);
 router.use(adminLogMiddleware);
+router.use(adminHistogramMiddleware);
 router.get('/users', async (req, res, next) => {
   try {
     const { limit, offset } = parsePagination(req.query as Record<string, string>);