Add CometTracer (#13661)
Hi! I'm Alex, Python SDK Team Lead from
[Comet](https://www.comet.com/site/).
This PR contains our new integration between langchain and Comet -
`CometTracer` class which uses new `comet_llm` python package for
submitting data to Comet.
No additional dependencies for the langchain package are required
directly, but if the user wants to use `CometTracer`, `comet-llm>=2.0.0`
should be installed. Otherwise an exception will be raised from
`CometTracer.__init__`.
A test for the feature is included.
There is also an already existing callback (and .ipynb file with
example) which ideally should be deprecated in favor of a new tracer. I
wasn't sure how exactly you'd prefer to do it. For example we could open
a separate PR for that.
I'm open to your ideas :)
