fix(instrumentation-express): preserve matched route after handler

[Philipp-Perepelitsky]

Assisted-by: ChatGPT 5.6 Sol

Which problem is this PR solving?

Fixes #2884.

Express middleware that runs after a matched request handler can overwrite RPCMetadata.route with a less specific middleware route. This can cause the HTTP server span to report an incorrect http.route.

Short description of the changes

-Preserve the route set by a request handler when later middleware executes.
-Continue allowing middleware to update route metadata before a request handler has matched.
-Add a regression test covering a /users/:id request handler followed by /users middleware.

Testing

-npm test -- --grep "rpcMetadata.route|should not overwrite route metadata" — 5 passing
-npm run compile — passing
-git diff --check — passing

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Philipp-Perepelitsky / name: Philipp-Perepelitsky (70ee6ef)

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