feat: implement LangfuseTracer

[italojohnny]

This PR implements the Langfuse trace service using the low-level SDK.

[github-actions]

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_{Last Modified at 16 Aug 24 21:13 UTC}

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[tianzhipeng-git]

Sorry for commenting on a closed PR, but I’m a bit confused about this:

1. Why did get_langchain_callback return None?
2. I only configured the LANGFUSE server, but I noticed there are traces were being sent to the LANGFUSE server via the langwatch callback. Any thoughts on this?

Thanks

@italojohnny @ogabrielluiz

[tianzhipeng-git]

Sorry for commenting on a closed PR, but I’m a bit confused about this:

1. Why did get_langchain_callback return None?
2. I only configured the LANGFUSE server, but I noticed there are traces were being sent to the LANGFUSE server via the langwatch callback. Any thoughts on this?

Thanks

@italojohnny @ogabrielluiz

I found the root cause of these problems, describe in #6746