docs: add per-route logging for Google Cloud Functions - #6907

Closes #4670.

Adds a per-route logging section to the Google Cloud Functions part of the serverless guide.

The section shows how to keep a single Fastify function while getting per-route visibility in Cloud Logging:

• emit a structured onResponse log with the matched route pattern
• use request.routeOptions.url so /users/123 and /users/456 group under /users/:id
• build log-based metrics from jsonPayload.route

The same hook applies to Firebase Functions with onRequest.

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