fix(instrumentation-fetch): do not modify the returned type of fetch

[dyladan]

fetch returns a native promise. If we wrap that promise, any code that checks if a promise is native will fail. Notably this happens in the case of WebAssembly.compileStreaming

fetch returns a Promise<Response>. When we wrap fetch we also wrap the Response. It used to be wrapped in a new Response object. Recently in #6343 a Proxy was introduced so that certain readonly properties could be retained.

This fix works by removing the Proxy and returning the original Response. The downside is that if the user cancels the request reader with reader.cancel(), our clone will not be cancelled and the clone will continue to read until the underlying stream is closed (most likely by the server finishing the response). AbortController.abort() still works because it aborts the underlying fetch, closing both the original and clone streams immediately, which is the typical way to abort a fetch anyway.

Fixes #6518

[codecov]

Codecov Report

❌ Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.67%. Comparing base (b5a6b12) to head (a491fc0).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	56.25%	7 Missing ⚠️

❌ Your patch status has failed because the patch coverage (56.25%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6521      +/-   ##
==========================================
- Coverage   95.73%   95.67%   -0.07%     
==========================================
  Files         364      364              
  Lines       12104    12069      -35     
  Branches     2887     2876      -11     
==========================================
- Hits        11588    11547      -41     
- Misses        516      522       +6     

Files with missing lines	Coverage Δ
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	85.84% <56.25%> (-4.20%)	⬇️

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[dyladan]

new Promise not required because context.with returns its callback return value synchronously, which is already a Promise

[raphael-theriault-swi]

LGTM, personally got no objection to the cancellation stuff. I'd still like a 2nd review from someone else with more familiarity with this code is possible though.

[dyladan]

@overbalance updated the PR. PTAL

[overbalance]

A runtime guard on error.status is probably overkill. Looks good!