Update B3 headers to lowercase format following spec recommendation

[leedh2004]

Hi maintainers,
I've updated the B3 propagator to use lowercase header names as recommended by the B3 specification.

The problem

The B3 specification states: When encoding in case-sensitive transports, prefer lowercase keys or the single header encoding which is explicitly lowercase.

• https://github.com/openzipkin/b3-propagation

Currently, Swift uses uppercase format (X-B3-TraceId, X-B3-SpanId, X-B3-Sampled) while several other language implementations (JavaScript, Rust, Python, etc.) already use lowercase.

What I changed

Updated B3 multi-header format to use lowercase:

• X-B3-TraceId → x-b3-traceid
• X-B3-SpanId → x-b3-spanid
• X-B3-Sampled → x-b3-sampled

Compatibility

• The B3 specification requires case-insensitive extraction, so this change maintains backward compatibility
• Other language implementations handle both cases during extraction
• This aligns Swift with the specification's preference for lowercase

Reference

• feat(b3-propagator)!: implement case-insensitive carrier handling opentelemetry-js#5859 (comment)
• https://github.com/open-telemetry/opentelemetry-js/blob/d3686b813cd2b0575eee44e0db020a043e9d510a/packages/opentelemetry-propagator-b3/src/constants.ts#L20

Let me know if you need any changes. Thanks!

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: leedh2004 / name: Dohyun Lee (46e9784, ed79db0)

[leedh2004]

I’ll move this PR to https://github.com/open-telemetry/opentelemetry-swift-core