This is a very speculative PR, with no tests or types yet. I'm interested what the view is from the maintainers about the use-case and the approach.

Fixes #3840

From that issue:

I'm keen to implement good logging and monitoring for my SvelteKit app. For monitoring, in particular, it's extremely useful to know which route, and which params, produced the final response. I use Prometheus, and with my old Express.js app I'd typically label each finished request/response with details like {method: 'GET', route: '/users/[id]/details', status: 200}. This generic route parameter means that I can aggregate all similar requests to the same route and understand how long the responses took to generate on average (or stddev, etc). At present, since I don't know which route rendered the response in handle(), the best I can do is to use url.pathname, which means I can't aggregate across similar request to the same route.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.

Tests

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