BUG: `setHeaders` throws when adding second `Server-Timing`-header (should be appended)

[tolu]

Describe the problem

I really like observability and traceability in chrome dev-tools and as such I fancy the Server-Timing header.

I noticed that setHeaders from getRequestEvent() fails here when attempting to set multiple Server-Timing headers. This is hard to get around since I set the headers in the code where async stuff happens

# error in terminal
Error: "Server-Timing" header is already set
    at setHeaders (node_modules/@sveltejs/kit/src/runtime/server/respond.js, <anonymous>:147:17)
    ...

This, given the current implementation, is expected behaviour since Server-Timing should be appended rather than set.

Describe the proposed solution

An even better solution would ofc be if setHeaders handled Server-Timing like .append in the implementation (perhaps several adapters, my usecase is node/deno).

If fixing this aligns with your thoughts, I'd be happy to provide a PR 🙏

Alternatives considered

I've worked around the problem by doing something like this:

/** somewhere in the code */
setHeaders({
	[`x-timing-${randomUUID()}`]: `${timingName};dur=${time}`,
});

/** server.hooks.ts */
response.headers.keys().filter((k) => k.startsWith('x-timing-')).forEach((k) => {
	const v = response.headers.get(k);
	response.headers.append('Server-Timing', v!);
	response.headers.delete(k);
});
response.headers.append('Server-Timing', `total;dur=${Date.now() - start}`);

Importance

would make my life easier

Additional Information

Thanks for this amazingly fun framework to work with 🫶