fix: carry the tracing span in the request store instead of copying the event

[Nic-Polumeyv]

Fixes #15040. Every server-side copy of the request event existed only to vary tracing.current per span. The span now lives in the request store and event.tracing.current reads it through a getter, the shape floated in #13899 (comment). The AsyncLocalStorage cost raised there is already paid, since every copy site was already wrapping its scope in a per-span store. Part of #16519.

---

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
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from d46938d:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/d46938d4fcdf68bb30adfb306440e12dc57a9960

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16600

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: d46938d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR