[o11y] Merge PipelineTracer with downstream version

[fhanau]

Also see the downstream PR.

@mar-cf I'm not sure if this is actually an improvement (the workerd version becomes more convoluted), Kenton suggested this change as the current approach violates KJ style

[fhanau]

This thing is not great – note that it is not newly added in this PR, but just ported from downstream. Last time I tested removing this it seemed to not break tests, I can try removing it in a future PR.

[mar-cf]

@kentonv could you clarify your suggestion to PipelineTracer? Is it in your opinion that it's cleaner if we move all we can into workerd? How should we handle internal repo specific behavior? Is there some prior art we could follow?

[kentonv]

@mar-cf

Is it in your opinion that it's cleaner if we move all we can into workerd?

No, my suggestion was the opposite: Move PipelineTracer into the internal codebase.

It is not meaningfully referenced anywhere in workerd. The only trick is that WorkerTracer contains kj::Maybe<kj::Rc<PipelineTracer>> parentPipeline;, but this is just an opaque reference that is held only to keep the object alive, so it could be changed to kj::Maybe<kj::Own<void>> instead.

[fhanau]

No, my suggestion was the opposite: Move PipelineTracer into the internal codebase.

It is not meaningfully referenced anywhere in workerd. The only trick is that WorkerTracer contains kj::Maybe<kj::Rc<PipelineTracer>> parentPipeline;, but this is just an opaque reference that is held only to keep the object alive, so it could be changed to kj::Maybe<kj::Own<void>> instead.

This no longer applies now that workerd supports tail workers: If tracing is set up, server/server.c++ creates a PipelineTracer and uses it to set up WorkerTracers and the streaming/legacy tail workers that have been configured. It might be possible to refactor the implementation to go without PipelineTracer, but getting memory management right and replacing the behavior of onComplete() would make this a more difficult task.

[mar-cf]

This might benefit from a quick sync to align on the goal and cover some implementation obstacles. We can progress faster, or decide it's more hassle than worth, if we're on the same page.

[fhanau]

Superseded by #5519.