fix: anchor FastAPI trace exclusions and honor the exclusion environment variables

[lesnik512]

Why

Unanchored FastAPI exclusions. FastAPI handed FastAPIInstrumentor the derived metrics and health paths as bare strings. OpenTelemetry's ExcludeList searches each pattern anywhere in the full URL, so a route merely containing one was silently left untraced. With 1.12.0, /metrics also excluded /api/metrics-report, and /custom-metrics/ excludes /api/custom-metrics/report. This is the same class of bug #248 fixed for Litestar.

Ignored environment variables. FastAPIInstrumentor and FastMCP's OpenTelemetryMiddleware read OTEL_PYTHON_FASTAPI_EXCLUDED_URLS / OTEL_PYTHON_STARLETTE_EXCLUDED_URLS (falling back to OTEL_PYTHON_EXCLUDED_URLS) only when no exclude list is passed. lite-bootstrap always passes one, so the variables never worked. Litestar honors its variable since #279.

Changes

• FastMCP's anchoring moves to OpenTelemetryInstrument._build_excluded_url_patterns(environment_key), and FastAPI uses it too:
  • Derived paths become ^\w+://[^/]*<path>(?:/|$), with the trailing slash optional, so they exclude both slash forms but no lookalike or sub-path.
  • opentelemetry_excluded_urls and the environment variable entries pass through verbatim, because they are regexes.
  • The environment variables are read the way OpenTelemetry reads them: the framework-specific one, else the generic one.
• Litestar keeps its own path-based patterns from feat!: trace Litestar through its own OpenTelemetryPlugin #279; its plugin already reads OTEL_PYTHON_LITESTAR_EXCLUDED_URLS.
• _build_excluded_urls is unchanged: ADR-0002 cites it, and the FastStream tests pin it.
• I checked that the URL ExcludeList sees has no query string, from asgi 0.49b0 through the current release, so the $ anchor is enough.

No config changes. Your own opentelemetry_excluded_urls behave as before; only the derived paths stop over-matching.

Tests

• FastAPI, regression: /custom-metrics/ and /custom-health/ produce no span, while /api/custom-metrics/report and /custom-healthy do. This test failed before the fix.
• FastAPI: each of OTEL_PYTHON_FASTAPI_EXCLUDED_URLS and OTEL_PYTHON_EXCLUDED_URLS excludes a route.
• FastMCP: OTEL_PYTHON_STARLETTE_EXCLUDED_URLS excludes POST /mcp.
• just lint, ty 0.0.85 and just test-ci pass: 362 passed, 100% coverage.