[metrics] instrument fastapi for tracing

[lun-4]

Motivation

I was checking tracing for our setup and noticed that fastapi is not instrumented: #10804 (comment), this leads to being unable to use sglang in other services that do use specific conventions for tracing (such as w3c's trace context, also used by opentelemetry, see comment for references). this was tested manually in our go net/http + otelhttp.Transport calling the sglang deployment.

Modifications

• add instrumentation for fastapi for optional tracing

Accuracy Tests

N/A

Benchmarking and Profiling

N/A

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
  • N/A?
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
  • N/A

[gemini-code-assist]

Summary of Changes

Hello @lun-4, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates OpenTelemetry tracing capabilities into the FastAPI server component of the sglang project. This enhancement ensures that requests processed by the FastAPI application are properly traced, enabling seamless integration with distributed tracing systems that adhere to standards like W3C Trace Context. The change addresses a critical gap in the existing tracing setup, allowing sglang to be used effectively in environments with comprehensive tracing requirements.

Highlights

• FastAPI Instrumentation: Introduced OpenTelemetry instrumentation for the FastAPI server to enable proper distributed tracing for requests.
• Dependency Update: Added opentelemetry-instrumentation-fastapi to the project's tracing dependencies and documentation.
• Tracing Integration: Implemented a new instrument_fastapi function in trace.py and integrated it into http_server.py to conditionally apply instrumentation to the FastAPI application.
• Motivation: This change addresses a previous issue where FastAPI was not instrumented, preventing the use of standard tracing conventions like W3C's trace context and OpenTelemetry in other services.

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[gemini-code-assist]

Code Review

This pull request adds optional FastAPI instrumentation for tracing by integrating opentelemetry-instrumentation-fastapi. The changes include updating dependencies, documentation, and adding the necessary code to instrument the FastAPI application.

My review has identified a critical issue in python/sglang/srt/entrypoints/http_server.py where the instrumentation is called at module import time, before tracing is initialized. This prevents the feature from working. I have provided a detailed comment on how to fix this by moving the instrumentation call into the server's lifespan context manager. Once this is addressed, the changes should correctly enable tracing for FastAPI requests.