Include B3 debug header in multi-header propagation fields

[kamikazeayoup]

Problem

The B3 multi-header propagator injects X-B3-Flags for debug requests but omits it from fields().

The propagator documentation instructs callers to clear these fields before reusing a carrier. Because the debug header is missing from the list, it survives cleanup and can cause a subsequent unsampled request to be treated as sampled.

Changes

Add B3Propagator.DEBUG_HEADER to FIELDS so callers can clear it before reusing a carrier.

Update fieldsList_multiInject to include the debug header and add inject_ReusedCarrier_DoesNotRetainDebugFlag to verify that a reused carrier does not retain the previous request’s debug flag.

Validation

• ./gradlew :extensions:trace-propagators:check passed locally.
• ./gradlew nativeTest --no-configuration-cache --no-parallel passed locally with Oracle GraalVM 25.0.4+7.1, the same version used by CI.

The GraalVM 25 compilation error reported in CI did not reproduce in this local run.

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Waiting on the author · refreshed 2026-10-12 19:11 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.65%. Comparing base (387f5c5) to head (1309335).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8930   +/-   ##
=========================================
  Coverage     90.65%   90.65%           
  Complexity    10887    10887           
=========================================
  Files          1015     1015           
  Lines         29731    29732    +1     
  Branches       3876     3876           
=========================================
+ Hits          26953    26954    +1     
  Misses         1860     1860           
  Partials        918      918           

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