fix(metrics): collapse static-asset routes to bounded "static" label (#362)

Follow-up to #355: http_request_duration_seconds's `route` label captured raw
content-hashed asset filenames (route="/assets/index-CAbxDtto.js",
"/assets/chunk-*.js"). @fastify/static serves each file through a route whose
matched routeOptions.url IS the raw hashed path, so the label was unbounded — a
new set of names every deploy, growing the series forever (the exact cardinality
leak the API routes were protected against).

resolveRouteLabel now detects a static request by its path prefix (/assets/,
/vad/, /brand/, /locales/) FIRST and collapses it to a single `static` label
(query string stripped before the check); API routes still use the template and
404s still collapse to `unknown`. Static edge latency is already measured by
Traefik's traefik_router_request_duration_*.

Gate: server tsc 0; metrics.spec passes (added static-collapse + query-strip +
"real API route mentioning assets is NOT collapsed" cases).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/metrics/http-metrics.hook.ts

@@ -2,15 +2,28 @@ import { FastifyReply, FastifyRequest } from 'fastify';
 import { isStreamingResponse } from './metrics.constants';
 import { observeHttp } from './metrics.registry';
 
+// URL path prefixes served by @fastify/static (client build output under
+// client/dist). Their filenames are content-hashed (index-*.js, chunk-*.js),
+// so a NEW set of names is minted on every deploy — using them as the `route`
+// label would grow the label set without bound (#362). Collapse them all to one
+// bounded `static` label. (Edge latency for static is already measured by
+// Traefik's traefik_router_request_duration_*.)
+const STATIC_PATH_PREFIXES = ['/assets/', '/vad/', '/brand/', '/locales/'];
+
 /**
  * Resolve the BOUNDED route label for an HTTP response.
  *
- * HARD REQUIREMENT (#355): use the ROUTE TEMPLATE (`/pages/:id`), NEVER the raw
- * URL (`/pages/abc-123`), so label cardinality stays finite. Fastify exposes the
- * matched template on `req.routeOptions.url`. On 404s (no route matched) that is
- * missing → collapse to the literal `unknown`.
+ * HARD REQUIREMENT (#355): use the ROUTE TEMPLATE (`/pages/:id`), NEVER a raw
+ * URL (`/pages/abc-123` or `/assets/index-CAbxDtto.js`), so label cardinality
+ * stays finite. Fastify exposes the matched template on `req.routeOptions.url`,
+ * BUT @fastify/static serves each file through a route whose matched url is the
+ * raw (hashed) file path — so for static assets that value is itself unbounded.
+ * Detect static requests by their path prefix FIRST and collapse to `static`;
+ * otherwise use the route template; on a 404 (no route matched) → `unknown`.
  */
 export function resolveRouteLabel(req: FastifyRequest): string {
+  const path = (req.url ?? '').split('?', 1)[0];
+  if (STATIC_PATH_PREFIXES.some((p) => path.startsWith(p))) return 'static';
   const url = req.routeOptions?.url;
   return typeof url === 'string' && url.length > 0 ? url : 'unknown';
 }

apps/server/src/integrations/metrics/metrics.spec.ts

@@ -25,6 +25,43 @@ describe('resolveRouteLabel (histogram route label)', () => {
     const req = { url: '/x' } as unknown as FastifyRequest;
     expect(resolveRouteLabel(req)).toBe('unknown');
   });
+
+  it.each([
+    '/assets/index-CAbxDtto.js',
+    '/assets/chunk-3OPIFGDE-CJOt9nr5.js',
+    '/assets/excalidraw-menu-DpsI0kFW.js',
+    '/vad/silero_vad_v5.onnx',
+    '/brand/logo.svg',
+    '/locales/en.json',
+  ])('collapses hashed/static asset %p to "static" (#362 cardinality)', (url) => {
+    // @fastify/static serves each file through a route whose matched url is the
+    // raw (hashed) file path, so routeOptions.url is itself unbounded here.
+    const req = {
+      url,
+      routeOptions: { url },
+    } as unknown as FastifyRequest;
+    const label = resolveRouteLabel(req);
+    expect(label).toBe('static');
+    expect(label).not.toContain('.js');
+    expect(label).not.toContain('index-');
+  });
+
+  it('strips the query string before the static-prefix check', () => {
+    const req = {
+      url: '/assets/index-CAbxDtto.js?v=2',
+      routeOptions: { url: '/assets/index-CAbxDtto.js' },
+    } as unknown as FastifyRequest;
+    expect(resolveRouteLabel(req)).toBe('static');
+  });
+
+  it('does NOT collapse a real API route that merely mentions assets', () => {
+    // A templated API route is kept as-is; only the static path PREFIXES collapse.
+    const req = {
+      url: '/api/pages/assets-guide',
+      routeOptions: { url: '/api/pages/:id' },
+    } as unknown as FastifyRequest;
+    expect(resolveRouteLabel(req)).toBe('/api/pages/:id');
+  });
 });
 
 describe('isStreamingResponse (SSE exclusion)', () => {