feat: add FastAPI middleware for per-request emissions tracking

[davidberenstein1957]

Summary

• Adds optional codecarbon[fastapi] integration with CodeCarbonMiddleware to measure CO₂ emissions per HTTP request across all FastAPI/Starlette routes
• Supports configurable response headers (presets, field lists, custom maps, or header_formatter callback) so clients can read emissions, energy, duration, and more from response headers
• Includes create_codecarbon_lifespan for shared app-level tracking, route-based task naming, path exclusions, and two tracking modes (request for concurrency-safe per-request trackers, app for lower-overhead serialized measurement)

Test plan

• uv run pytest tests/integrations/ -v — 17 tests pass
• Manual smoke: uv run --extra fastapi uvicorn examples.fastapi_middleware:app --reload then curl -i localhost:8000/predict
• Verify docs build: uv run task docs

Usage

from fastapi import FastAPI
from codecarbon.integrations.fastapi import add_codecarbon_middleware

app = FastAPI()
add_codecarbon_middleware(app, project_name="my-api", response_headers="default")

See docs/how-to/fastapi.md for full configuration options.

Made with Cursor

[codecov]

Codecov Report

❌ Patch coverage is 94.68599% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.17%. Comparing base (d2ce064) to head (bb6995c).

Files with missing lines	Patch %	Lines
codecarbon/integrations/fastapi/middleware.py	91.89%	9 Missing ⚠️
codecarbon/integrations/fastapi/_routing.py	95.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1203      +/-   ##
==========================================
+ Coverage   88.88%   89.17%   +0.28%     
==========================================
  Files          45       50       +5     
  Lines        4302     4509     +207     
==========================================
+ Hits         3824     4021     +197     
- Misses        478      488      +10     

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[benoit-cty]

Hello, thanks for this. There is a problem with your branch : there are many changes that are already merged. Can you do a rebase ?

[davidberenstein1957]

@benoit-cty I have reached out to some people at FastAPI, if they would be interested in a quick review :) For visibility, we could also consider deploying it as a standalone integration, but let's see if people like it.