Add LangChain instrumentation with OTel GenAI semantic convention sup…

[jimilp7]

This PR adds a new logfire.instrument_langchain() method that provides callback-based instrumentation for LangChain, capturing the complete execution hierarchy.

Features

• Full execution hierarchy: Chains, tools, retrievers, and LLM calls
• Tool definitions: Captures gen_ai.tool.definitions from LLM invocations
• OTel GenAI messages: Input/output messages in OTel format
• System instructions: Extracted and formatted per OTel spec
• Conversation tracking: Via thread_id -> gen_ai.conversation.id
• SpanKind.CLIENT: LLM calls use CLIENT kind for external service calls

Usage:

import logfire
from langgraph.prebuilt import create_react_agent

logfire.configure()
logfire.instrument_langchain()

agent = create_react_agent(model, tools)
agent.invoke({"messages": [...]})

[codecov]

Codecov Report

❌ Patch coverage is 32.76836% with 238 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
logfire/_internal/integrations/langchain.py	25.00%	209 Missing and 1 partial ⚠️
logfire/_internal/exporters/processor_wrapper.py	49.01%	20 Missing and 6 partials ⚠️
logfire/_internal/main.py	89.47%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[jimilp7]

@alexmojaki I'd love to get your initial thoughts on this approach for langchain, if it makes sense to you, I can proceed with creating tests. Basically, langgraph native instrumentation does not follow semconv, so we use callback-based approach to manually instrument/process into semconv.

[jimilp7]

@alexmojaki Any updates on this?

@alexmojaki I'd love to get your initial thoughts on this approach for langchain, if it makes sense to you, I can proceed with creating tests. Basically, langgraph native instrumentation does not follow semconv, so we use callback-based approach to manually instrument/process into semconv.

[alexmojaki]

I'm reluctant to increase the amount of stuff to maintain. Do you think it's possible to get this into the openinference package we use?