fix(otlp-exporter-base): remove sendBeacon in favor of fetch with keepalive

[overbalance]

Which problem is this PR solving?

TLDR sendBeacon uses fetch+keepalive internally

navigator.sendBeacon() cannot see HTTP response codes, it returns whether the request was queued, not whether the server accepted it:

const queued = navigator.sendBeacon(url, data);
// queued === true, but server returned 503 → data silently lost

This means server errors (503, 429) cause silent data loss with no opportunity to retry. The RetryingTransport wrapper expects a 'retryable' status to trigger retries, but sendBeacon could only return 'success' or 'failure', breaking the retry logic entirely.

Additionally, when the browser's cumulative 64KB limit or 9 concurrent request limit is exceeded, sendBeacon rejects the request outright by returning false. There's no way to fall back to a non-keepalive request that would still deliver the data.

Fixes #3489
Fixes #6274

Short description of the changes

Replace sendBeacon transport with fetch + keepalive and track pending requests to stay within browser limits:

• 60KB cumulative body size (headroom below 64KB browser limit)
• 9 concurrent keepalive requests (Chrome limit)
• Graceful degradation: When limits exceeded, disable keepalive (request still works, just won't survive page unload)
• Deprecate createOtlpSendBeaconExportDelegate: Still works but delegates to fetch transport

Impact

Scenario	Before	After
Server returns 503	Data silently lost	Retried via RetryingTransport
64KB limit exceeded	Request fails	Request succeeds without keepalive
Page unload	sendBeacon queues blindly	fetch+keepalive with limit tracking

Migration

For SDK users: No action required. Behavior improves automatically.

For custom transports: If using createOtlpSendBeaconExportDelegate, it still works but is deprecated. Switch to createOtlpFetchExportDelegate when convenient.

References

• WHATWG Fetch #679: Keepalive limits (64KiB cumulative, 9 concurrent)
• Huli's Blog: 64KiB Limitation

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• npm run compile && npm test passes
• npm run test:browser passes
• Queue tracking: keepalive enabled/disabled based on limits
• Retry integration: 503/429 retried, 404 not retried
• Counter cleanup after request completion

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (bfd06e6) to head (ae60364).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6391      +/-   ##
==========================================
+ Coverage   95.50%   95.59%   +0.08%     
==========================================
  Files         365      361       -4     
  Lines       11617    11595      -22     
  Branches     2681     2683       +2     
==========================================
- Hits        11095    11084      -11     
+ Misses        522      511      -11     

Files with missing lines	Coverage Δ
...tlp-exporter-base/src/transport/fetch-transport.ts	94.64% <100.00%> (+2.97%)	⬆️

... and 3 files with indirect coverage changes

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[overbalance]

Typo seemed within scope.

[overbalance]

This applies to all requests per renderer, so not just OTel requests from my understanding. I don't think there's a better solution here but am open to ideas.

[pichlermarc]

this looks good. once this is merged, we should also add an issue to remove the deprecated code eventually.

Would appreciate one additional review from @open-telemetry/browser-approvers before merging this :)

Edit: this probably supersedes #6358

[pichlermarc]

deprecations are always weird to categorize. I usually just put a blip here to let people know what's coming. (feel free to ignore)

[overbalance]

Added

[david-luna]

looks good. A comment that you can skip if you want