handleResponse hook for logging/monitoring which route was matched

[georgecrawford]

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.

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[Rich-Harris]

It took a while, but we're finally in a position to address this issue. A new hook isn't ideal, and nor is exposing the internal route object, but by getting rid of fallthrough routes (#4330) we can determine the route before we even call handle, which means we can expose the internal route.key property on the event, which means you can do the logging inside handle.

Closing in favour of #4345