feat: Add support for Langchain Callbacks

[Xabilahu]

Solves Issue https://github.com/traceloop/openllmetry-js/issues/133: 🚀 Feature: re-write Langchain instrumentation to use Langchain Callbacks

⚠️ This PR is actually in WIP status until we find a way to monkey patch class constructors ⚠️

Implemented a BaseCallbackHandler (source for the interface). The main idea is that the handler has some state for each of the spans it starts, so that when handleXXEnd or handleXXError are invoked, the same span can be properly ended. To do that, we are using a stack, the logic is as follows:

1. We start a Span whenever handleXXStart is called and push it into the stack.
2. If handleXXEnd is called:
   1. If there are no Span instances in the stack, we throw an Error, as handleXXEnd cannot be called before the corresponding handleXXStart.
   2. Otherwise, we pop the Span from the stack, and end it accordingly.
3. If handleXXError is called, we follow a similar logic as for handleXXEnd.

This approach has the benefit of being able to properly handle nested spans.

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Tests

Unit tests

npm run build:all && npx nx run @traceloop/instrumentation-langchain:test

Integration tests (aka sample-app)

• Modified the sample-app for LangChain (this is expected to go away before merging this PR, as it's just for my own testing purposes) to:
  • Not depend on OpenAI stuff, as I lack an API key 🥲
  • Make it faster by just running a single workflow
  • Disable manual instrumentation of LangChain modules
  • Instantiate and pass the TraceloopCallbackHandler to check if the spans are generated properly
• Start Ollama serving llama2 LLM by running:

docker run -d -v ollama:/root/.ollama -p 11434:11434 --name ollama ollama/ollama && docker exec -it ollama ollama run llama2

• Ran the sample app via:

TRACELOOP_API_KEY=<REDACTED> npx nx run sample-app:run:langchain

• Verified that there is a trace in my Traceloop development stage, but it lacks the spans from the CallbackHandler, as I'm using a ProxyTracerProvider, which defaults to NoOp tracer when no delegate is provided. This behavior should change, and spans will be sent once we manage to mockey-patch LangChain constructors, and inject the CallbackHandler in constructor params.
• For the moment, I just verified the behavior via logs.

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[nirga]

@Xabilahu just for my own records, linking our discussion from Slack where I suggested a solution to the problems you've raised here.