feat: add content-length headers to generated responses, via new `text(...)` helper

[Rich-Harris]

closes #5749.

This is just an idea; I don't know if it's the right one. In order to make it easier to determine response content length inside handle, for logging purposes, this adds a new text(body) helper similar to the json(data) helper. It returns a Response object with a content-length header, and uses it in favour of new Response(...) when generating pages and other responses.

One observation: it'd be nice if we could unit-test Server rather than having Playwright tests for the bulk of the tests in server.test.js. Might look into that separately.

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[Rich-Harris]

Ah, that's a good point. I would have assumed that if something in between SvelteKit and the user were to compress the body it would generate a new response with new headers, but I'll freely confess my ignorance here. Safest thing would be to close #5749 as wontfix but that seems unsatisfying.

Am inclined to just leave this here until someone with more authority weighs in

[bjon]

@dummdidumm Google's dynamic compression (CDN) requires the response to have content-length. Without it, no compression is done. https://cloud.google.com/cdn/docs/dynamic-compression#response-not-compressed

It's was no problem to add content-length in sveltekit manually. Maybe the text() and json() could have an optional addContentLength: boolean arg, or something similar? It would be great to have kit handle it.

[Conduitry]

If there's some library or something that compresses responses on the fly but doesn't correctly update the content-length header, that's certainly a bug in that library. I think we should only worry about that if there were some extremely popular library that did that that seemed extremely unlikely to want to fix that bug.

[Rich-Harris]

I don't think there's much value in having an addContentLength option — it doesn't cost anything extra for us, and I can't imagine any cases where you wouldn't want the header. Honestly I'm a little surprised it doesn't get added automatically (if you do new Response('hello') it'll add a content-type: text/plain;charset=UTF-8 header, but not content-length — what the hell?)

So it sounds like the consensus is in favour of merging this?