feat: Add W3C traceparent support to tracing client and utils

[hangy]

📜 Description

Based on the changes in getsentry/sentry-php@a31d418, passes the Sentry traceId and spanId as a W3C traceparent header in HTTP requests to allow trace propagation using W3C standards.

💡 Motivation and Context

This adds support for propagating the Sentry TraceId as a W3C TraceSpan header to support #1831.

This is probably the easiest way to implement W3C traceparent headers without introducing breaking changes. There might be a performance implication as the header value is recomputed every time. As an alternative, I considered adding a computed value to SentryTraceHeader (akin to SentryTraceHeader.value), but that would violate that class' responsibility. Another alternative might be adding a ISentrySpan.toW3CTrace() method, but introducing this as a breaking change feels kinda clunky.

💚 How did you test it?

I currently don't have a full test setup for dart, so I tested with integration tests only for now.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed - the referenced change of the PHP SDK didn't introduce any doc updates.
• All tests passing
• No breaking changes

🔮 Next steps

If anyone had a Sentry integrated dart client that called a server that supports the W3C traceparent header, it would be great if they could test this PR.

[buenaflor]

hey, thanks for the PR, changes look good to me overall but during the review I noticed a bug in the tracing client (unrelated to this PR) which I'd like to fix first before adding this

[buenaflor]

thx for the contribution!

looks good to me overall, merging this will be a bit later after we released v9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.64%. Comparing base (886a258) to head (6308b4f).
Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2843      +/-   ##
==========================================
- Coverage   87.68%   87.64%   -0.05%     
==========================================
  Files         272      272              
  Lines        9031     9058      +27     
==========================================
+ Hits         7919     7939      +20     
- Misses       1112     1119       +7     

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