fix: distributed propagation

[vabruzzo]

Bugfix for distributed propagation

Key Changes:

• Uses w3c_propagator for distributed trace run continuation

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Generated Summary:

• Introduced handling for cases when the global propagator is a NoExtract instance.
• Added a fallback mechanism to use the W3C propagator directly with TraceContextTextMapPropagator.
• Modified context attachment logic to ensure it only occurs if the trace_id from the span context is valid.
• Implemented a clearer fallback behavior by creating a new span if the provided context is invalid.
• These changes enhance the robustness of tracing context propagation, ensuring that traces can continue under more conditions.

This summary was generated with ❤️ by rigging

[vabruzzo]

Question for @monoxgas - We utilize the W3C propagator to fix this issue. Should we make this the default global propagator? My inclination is yes, and based on research it seems like that would work fine, but I'm not sure if you've run into anything regarding this issue for non distributed runs?

[vabruzzo]

i think we can just always use this globally rather than switching it out here

[monoxgas]

Like these changes, if I'm understanding correctly this is all a consequence of:

1 - When distributed=False on logfire, it overrides the global textmap provider to a NoExtract variant: https://github.com/pydantic/logfire/blob/e0d3fdff5ce2f2490f62529d47dd12dc58be51ff/logfire/_internal/config.py#L1126C31-L1126C49
2 - This doesn't affect the injection, so we capture context properly, but when we go to use it on the far side it doesn't actually pull out the context we need.

I like these changes, inclined to keep our override as a check for NoExtract. It's similar to how logfire does it on their end: https://github.com/pydantic/logfire/blob/e0d3fdff5ce2f2490f62529d47dd12dc58be51ff/logfire/propagate.py#L68

As an extension at some point, I think we should expose the distributed setting to the logfire.configure call in our settings object and set it to True by default.

[vabruzzo]

yep that my understanding is aligned here. Thanks