Tag worker events with prediction (#2020)
* Formatting, moving `Envelope`, and using "tag"
* Include tag in Cancel event
and clean up TODO comments
* Add pydantic v2 support to child worker `_apredict`
* Keep a map of thread IDs to tags on _ChildWorker
* Use context var to pass tag instead of dict
since context vars are meant to be async-safe
* don't use the contextvar in sync StreamRedirector
* fix StreamPredictor working with tags
* fix record_event with tags
We were unintentionally dropping the tag from record_event() with sync
predictions.
This is fixed by introducing a self._current_tag property which Does The Right
Thing whether in sync or async context.
As part of this, we can also go back to a single _stream_write_hook()
implementation.
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Co-authored-by: Dominic Baggott <dominic.baggott@gmail.com>
Co-authored-by: Philip Potter <phil@replicate.com>
