feat(b3-propagator)!: implement case-insensitive carrier handling

[YangJonghun]

Which problem is this PR solving?

This PR addresses compatibility issues with B3 propagator carrier handling across different language implementations. While other OTel language implementations use standardized B3 formats, the JavaScript implementation uses lowercase format, causing interoperability issues when traces cross language boundaries or when using non-HTTP transport methods.

Short description of the changes

• Add fallback to lowercase header lookup in getHeaderValue function
• Standardize B3 constants to proper case format (X-B3-*)
• Improve compatibility with case-varying HTTP implementations

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
  • Injection now uses proper case format (X-B3-* instead of x-b3-*).
  • Extract handles both cases for backward compatibility.

How Has This Been Tested?

• Existing unit tests continue to pass
• Manual testing with different case combinations

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 95.03%. Comparing base (8d0dcea) to head (ea432d7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5859   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files         306      306           
  Lines        7969     7969           
  Branches     1611     1612    +1     
=======================================
  Hits         7573     7573           
  Misses        396      396           

Files with missing lines	Coverage Δ
...entelemetry-propagator-b3/src/B3MultiPropagator.ts	100.00% <100.00%> (ø)
...kages/opentelemetry-propagator-b3/src/constants.ts	100.00% <100.00%> (ø)

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[pichlermarc]

+1 for extracting in a case insenstivie way from the carrier, but IIUC that should be done with a corresponding TextMapGetter for each instrumentation. For inejcting I'd prefer lowercase. From the B3 spec:

Note: Http headers are case-insensitive, but sometimes this encoding is used for other transports. When encoding in case-sensitive transports, prefer lowercase keys or the single header encoding which is explicitly lowercase.

Can you give an example which setup caused problems for you?

[YangJonghun]

@pichlermarc
Thank you for the review.
I missed that part of the spec, and as you pointed out, it looks correct to remove the inject logic change and only handle extraction in a case-insensitive way. I appreciate you pointing that out.

I ran into issues when propagating trace context between Native (Android, iOS) and a WebView. In Swift and Java, the OTel B3Propagator implementations define the B3-related constants with camelcase, so they don’t recognize lowercase propagation.
Looking through the OpenTelemetry source, I noticed that some languages handle lowercase while others don’t, so it would be nice if this could be unified across implementations. 🙏