[BUG]: missing trace headers on fetch requests when using `Request` object and `RequestInit` with headers

[anakorn-wm]

Tracer Version(s)

5.57.1

Node.js Version(s)

22.17.0

Bug Report

When using the dd-trace fetch plugin, fetch requests made using a Request object and with headers on the RequestInit object result in HTTP requests with missing trace headers.

In comparison, fetch requests made with a string URL and with headers on the RequestInit object are working as expected, and trace headers are present on the resulting HTTP request.

import ddTrace from 'dd-trace';

ddTrace.init();
ddTrace.tracer.use('fetch');

const url = 'https://petstore.swagger.io/v2/pet/1';

const request = new Request(url);
const init = { headers: { foo: 'bar' } };

// trace headers are missing, e.g. `x-datadog-trace-id`, `x-datadog-parent-id`, `x-datadog-tags`
await fetch(request, init);

// trace headers are present
await fetch(url, init);

I'm also able to replicate the issue by swapping in a Request object in place of the string URL in the below test:

dd-trace-js/packages/datadog-plugin-fetch/test/index.spec.js

Line 217 in 3c9fc50

fetch(`http://localhost:${port}/user?foo=bar`, { headers: { foo: 'bar' } })

i.e. using this instead:

          const req = new globalThis.Request(`http://localhost:${port}/user?foo=bar`)
          fetch(req, { headers: { foo: 'bar' } })

I'm not very familiar with dd-trace-js code, but I'm able to fix the issue by removing this snippet from the fetch instrumentation code:

dd-trace-js/packages/datadog-instrumentations/src/helpers/fetch.js

Lines 15 to 19 in 669090c

	if (input instanceof Request) {
	const ctx = { req: input }
	return ch.tracePromise(() => fetch.call(this, input, init), ctx)
	}

But I realize this was added to address #4258 where errors were being produced when Request objects were being reused, so I'm not sure about the right path forward here.

Reproduction Code

(attached on bug report)

Error Logs

No response

Tracer Config

No response

Operating System

Darwin Kernel Version 24.5.0

Bundling

No Bundling