(feat) Add usage tracking for streaming `/anthropic` passthrough routes

Adds usage tracking + spend tracking + unit testing for pass through anthropic routes

Relevant issues

Type

🆕 New Feature 🐛 Bug Fix 🧹 Refactoring 📖 Documentation 🚄 Infrastructure ✅ Test

Changes

[REQUIRED] Testing - Attach a screenshot of any new tests passing locall

If UI changes, send a screenshot/GIF of working UI fixes

Description by Korbit AI

What change is being made?

Add usage tracking and logging for /anthropic passthrough routes in the proxy by introducing a convert_str_chunk_to_generic_chunk function and creating an AnthropicPassthroughLoggingHandler class, which processes Anthropic responses, constructs logging payloads, and integrates them into the existing streaming infrastructure.

Why are these changes being made?

To improve monitoring and understanding of Anthropic API usage by providing detailed logging, to align with existing logging patterns for other endpoints, and to enhance transparency and debugging capabilities for requests passing through the proxy. This refactoring aims to modularize and streamline the passthrough handling, making the logging process more efficient and maintainable.

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BerriAI / litellm