[RUMF-824] support Request instances in tracing

[BenoitZugmeyer]

Motivation

Support Request instances in tracing

fetch(
  new Request('http://localhost:3000', {
    headers: {
      'x-foo': 'bar',
    },
  })
)

Changes

Clone instance and add trace headers if the fetch init argument is a Request.

Testing

Unit

---

I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #684 (86b0105) into master (5415a6f) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
- Coverage   77.98%   77.85%   -0.13%     
==========================================
  Files          60       60              
  Lines        3284     3288       +4     
  Branches      714      715       +1     
==========================================
- Hits         2561     2560       -1     
- Misses        723      728       +5     

Impacted Files	Coverage Δ
packages/core/src/browser/fetchProxy.ts	97.72% <100.00%> (ø)
packages/rum-core/src/domain/tracing/tracer.ts	100.00% <100.00%> (ø)
packages/rum-core/src/transport/batch.ts	68.42% <0.00%> (-10.53%)	⬇️
...ckages/core/src/domain/automaticErrorCollection.ts	98.41% <0.00%> (-1.59%)	⬇️

---

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[bcaudan]

It seems a bit weird to me to not update input headers but to provide an init headers instead.
I am wondering if it could not be better to provide an updated input only:

context.input = new Request(request, {
  headers: headers.concat(objectEntries(tracingHeaders) as string[][])
})

no strong argument for one or the other though.
wdyt?

[BenoitZugmeyer]

Mmh it seems to be equivalent, since calling fetch(input, init) calls new Request(input, init) internally. After a few tests, I think that it should be fine. I only spoted an issue with streaming requests but it is likely to be a Chrome bug in the ongoing implementation:

const request = new Request('/post', {
  method: 'POST',
  body: new ReadableStream(),
  mode: 'cors'
});
// works:
await fetch(request, { mode: 'cors' })
// fails with: TypeError: Failed to construct 'Request': If request is made from ReadableStream, mode should be"same-origin" or "cors"
const requestCopy = new Request(request)
await fetch(requestCopy, { mode: 'cors' })

I think it's fine nonetheless, I updated the implementation, let me know if it looks better to you.

[bcaudan]

👍