Next.js instrumentation doesn't honour usePathAsTransactionName config var

[trentm]

As discussed at https://discuss.elastic.co/t/next-js-and-dynamic-routes/321302 the current Next.js instrumentation that sets the transaction.name for a Dynamic Route does not honour the usePathAsTransactionName: true config var. That means that a request GET /a-dynamic-route/42 to this dynamic route https://github.com/elastic/apm-agent-nodejs/blob/main/test/instrumentation/modules/next/a-nextjs-app/pages/a-dynamic-page/%5Bnum%5D.js results in transaction.name = 'GET /a-dynamic-route/[num] even if usePathAsTransactionName: true.

Note that there is also naming of the Next.js _data/route ... transactions for possibly dynamic routes -- e.g. "Next.js _next/data route /a-dynamic-page/[num]". However, I don't think those should honour usePathAsTransactionName. At least not without user feedback.

[trentm]

Note to self: a start at this is:

diff --git a/lib/instrumentation/modules/next/dist/server/dev/next-dev-server.js b/lib/instrumentation/modules/next/dist/server/dev/next-dev-server.js
index ab1e7052..d02b5da8 100644
--- a/lib/instrumentation/modules/next/dist/server/dev/next-dev-server.js
+++ b/lib/instrumentation/modules/next/dist/server/dev/next-dev-server.js
@@ -143,7 +143,11 @@ module.exports = function (mod, agent, { version, enabled }) {
       // specific route wrapper hasn't already done so.
       if (trans && !trans[kSetTransNameFn]) {
         trans[kSetTransNameFn] = (req, pathname) => {
-          trans.setDefaultName(`${req.method} ${pathname}`)
+          if (agent._conf.usePathAsTransactionName) {
+            trans.setDefaultName(`${req.method} ${req.url}`)
+          } else {
+            trans.setDefaultName(`${req.method} ${pathname}`)
+          }
           // Ensure only the first `findPageComponents` result sets the trans
           // name, otherwise a loaded `/_error` for page error handling could
           // incorrectly override.
diff --git a/lib/instrumentation/modules/next/dist/server/next-server.js b/lib/instrumentation/modules/next/dist/server/next-server.js
index 0431e260..428309fe 100644
--- a/lib/instrumentation/modules/next/dist/server/next-server.js
+++ b/lib/instrumentation/modules/next/dist/server/next-server.js
@@ -140,7 +140,11 @@ module.exports = function (mod, agent, { version, enabled }) {
       // specific route wrapper hasn't already done so.
       if (trans && !trans[kSetTransNameFn]) {
         trans[kSetTransNameFn] = (req, pathname) => {
-          trans.setDefaultName(`${req.method} ${pathname}`)
+          if (agent._conf.usePathAsTransactionName) {
+            trans.setDefaultName(`${req.method} ${req.url}`)
+          } else {
+            trans.setDefaultName(`${req.method} ${pathname}`)
+          }
           // Ensure only the first `findPageComponents` result sets the trans
           // name, otherwise a loaded `/_error` for page error handling could
           // incorrectly override.