Added support to get usage for ChatGoogleGenerativeAI

[NeerajG03]

This PR has been raised to add support to track ChatGoogleGenerativeAI's langchain call usage.
A simple check for usage_metadata attribute and then parsing using the _parse_usage_model

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[greptile-apps]

Disclaimer: Experimental PR review

PR Summary

Added support for tracking usage of ChatGoogleGenerativeAI's langchain calls in the Langfuse Python SDK.

• Introduced check for 'usage_metadata' attribute in message chunks in langfuse/callback/langchain.py
• Implemented parsing of 'usage_metadata' using _parse_usage_model function
• Enhanced usage tracking capabilities for Google's Generative AI in addition to existing LLM providers

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[maxdeichmann]

@NeerajG03 did you test this locally? Thanks a lot for the contribution!

[NeerajG03]

@maxdeichmann I'm sorry i'm very new to contributing for projects.
If you are asking if i have run this test poetry run pytest -s -v --log-cli-level=INFO, then thats a yes. 👍
I ran it for test_langchain.py and test_langchain_integration.py

[NeerajG03]

Here is the screenshot of the run summary