Fix router not working with deeply nested middleware (#5890)

Fixes: #5881

Author: BridgeAR

packages/datadog-instrumentations/src/router.js

@@ -106,7 +106,7 @@ function createWrapRouterMethod (name) {
   }
 
   function extractMatchers (fn) {
-    const arg = Array.isArray(fn) ? fn : [fn]
+    const arg = Array.isArray(fn) ? fn.flat(Infinity) : [fn]
 
     if (typeof arg[0] === 'function') {
       return []

packages/datadog-plugin-router/test/index.spec.js

@@ -111,7 +111,7 @@ describe('Plugin', () => {
           router.use((req, res, next) => {
             return next('route')
           })
-          router.get('/foo', (req, res) => {
+          router.get('/foo', [[[(_req, _res, next) => { next() }]]], (req, res) => {
             res.end()
           })