fix(node): Fix 404 route handling in express 5 (#16211)

Closes #16210

Author: mydea

dev-packages/node-integration-tests/suites/express-v5/tracing/test.ts

@@ -93,8 +93,7 @@ describe('express v5 tracing', () => {
       const runner = createRunner()
         .expect({
           transaction: {
-            // FIXME: This is wrong :(
-            transaction: 'GET /',
+            transaction: 'GET /does-not-exist',
             contexts: {
               trace: {
                 span_id: expect.stringMatching(/[a-f0-9]{16}/),
@@ -103,8 +102,6 @@ describe('express v5 tracing', () => {
                   'http.response.status_code': 404,
                   url: expect.stringMatching(/\/does-not-exist$/),
                   'http.method': 'GET',
-                  // FIXME: This is wrong :(
-                  'http.route': '/',
                   'http.url': expect.stringMatching(/\/does-not-exist$/),
                   'http.target': '/does-not-exist',
                 },

packages/node/src/integrations/tracing/express-v5/instrumentation.ts

@@ -164,16 +164,18 @@ export class ExpressInstrumentationV5 extends InstrumentationBase<ExpressInstrum
           // remove duplicate slashes to normalize route
           .replace(/\/{2,}/g, '/');
 
+        const actualRoute = route.length > 0 ? route : undefined;
+
         const attributes: Attributes = {
           // eslint-disable-next-line deprecation/deprecation
-          [SEMATTRS_HTTP_ROUTE]: route.length > 0 ? route : '/',
+          [SEMATTRS_HTTP_ROUTE]: actualRoute,
         };
         const metadata = getLayerMetadata(route, layer, layerPath);
         const type = metadata.attributes[AttributeNames.EXPRESS_TYPE] as ExpressLayerType;
 
         const rpcMetadata = getRPCMetadata(context.active());
         if (rpcMetadata?.type === RPCType.HTTP) {
-          rpcMetadata.route = route || '/';
+          rpcMetadata.route = actualRoute;
         }
 
         // verify against the config if the layer should be ignored