[Span First #1]: Add simple span API

[buenaflor]

📜 Description

Span First initiative

💡 Motivation and Context

Part of #3331

This is only basic API scaffolding

💚 How did you test it?

No tests yet since there is no specific logic yet

📝 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
• All tests passing
• No breaking changes

🔮 Next steps

#skip-changelog

[codecov]

Codecov Report

❌ Patch coverage is 5.88235% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.91%. Comparing base (bbdbcb9) to head (9219c7b).
⚠️ Report is 1 commits behind head on feat/span-first.

Files with missing lines	Patch %	Lines
packages/dart/lib/src/protocol/unset_span.dart	7.69%	12 Missing ⚠️
packages/dart/lib/src/protocol/noop_span.dart	0.00%	8 Missing ⚠️
packages/dart/lib/src/hub.dart	0.00%	6 Missing ⚠️
packages/dart/lib/src/hub_adapter.dart	0.00%	2 Missing ⚠️
packages/dart/lib/src/noop_hub.dart	0.00%	2 Missing ⚠️
packages/dart/lib/src/sentry.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           feat/span-first    #3360      +/-   ##
===================================================
- Coverage            88.31%   87.91%   -0.41%     
===================================================
  Files                  291      294       +3     
  Lines                 9957     9991      +34     
===================================================
- Hits                  8794     8784      -10     
- Misses                1163     1207      +44     

☔ View full report in Codecov by Sentry.
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[buenaflor]

Also open to calling this SpanImpl or something different. The gist is that this is the default span impl

[buenaflor]

As discussed we will change this to using an internal UnsetSpan which should serve as a marker for not provided span

[cursor]

Bug: Redundant conditional logic, both branches return same value

The startSpan method contains redundant conditional logic where all code paths return NoOpSpan(), making the if (!_isEnabled) and else if (_options.isTracingEnabled()) checks meaningless. When tracing is enabled, the method should perform actual implementation logic (as indicated by the TODO), but currently just returns the same no-op as disabled paths. Either the conditional branches are unnecessary, or the enabled branch should contain actual implementation.

 

[buenaflor]

This is just scaffolding, real impl will follow in the next PR