[CO] Add support for Express .use() routes (#6164)

Author: watson

packages/datadog-instrumentations/src/router.js

@@ -146,7 +146,7 @@ function createWrapRouterMethod (name) {
       }
 
       if (routeAddedChannel.hasSubscribers) {
-        routeAddedChannel.publish({ topOfStackFunc: methodWithTrace, layer: this.stack[0] })
+        routeAddedChannel.publish({ topOfStackFunc: methodWithTrace, layer: this.stack.at(-1) })
       }
 
       if (this.stack.length > offset) {

packages/datadog-plugin-express/test/code_origin.spec.js

@@ -87,6 +87,17 @@ describe('Plugin', () => {
               await assertCodeOrigin('/v1/user', { line, method: 'testCase', type: 'Context' })
             })
 
+            it('should support .use() routes', async function testCase () {
+              app.get('/route_before', (req, res) => res.end())
+              const line = getNextLineNumber()
+              app.use('/foo', (req, res) => {
+                res.end()
+              })
+              app.get('/route_after', (req, res) => res.end())
+
+              await assertCodeOrigin('/foo/bar', { line, method: 'testCase', type: 'Context' })
+            })
+
             it('should point to route handler even if passed through a middleware', async function testCase () {
               app.use((req, res, next) => {
                 next()