Anchor derived trace exclusions for FastAPI and FastMCP

[lesnik512]

Fixes #158 (part of #156). The Litestar half, #159, waits for #154, which rewrites the same Litestar exclusion code.

Motivation

OpenTelemetry's ExcludeList searches every pattern anywhere in the full URL. The default opentelemetry_exclude_urls = ["/metrics"], and the health-check path added when opentelemetry_generate_health_check_spans=False, therefore also silenced any route merely containing them: /api/metrics-report and /api/health/details produced no server span on FastAPI and FastMCP.

Changes

• The metrics path is derived from prometheus_metrics_path (newly mirrored on OpentelemetryConfig, the way health_checks_path already is) instead of being the default value of opentelemetry_exclude_urls, which now defaults to [].
• New define_excluded_paths() returns the paths the library derives: the metrics path, plus the health path when health spans are off.
• New define_excluded_url_patterns(), used by FastAPI and FastMCP, anchors those paths as ^\w+://[^/]*<path>(?:/|$), with the trailing slash optional. /metrics then excludes /metrics and /metrics/ but not /api/metrics or /metrics-report.
• User-supplied opentelemetry_exclude_urls entries are documented as regexes and stay verbatim.
• define_exclude_urls(), which Litestar uses, returns the same list as before for default settings. Litestar's behavior is unchanged, and the shared module does not conflict with Use Litestar's OpenTelemetryPlugin in the Litestar bootstrapper #154.
• README: both opentelemetry_exclude_urls descriptions updated.

Behavior changes

• The default value of opentelemetry_exclude_urls is [] instead of ["/metrics"]; the metrics route stays excluded by default.
• Setting opentelemetry_exclude_urls to a custom list used to drop the /metrics exclusion as a side effect. Now prometheus_metrics_path is always excluded, on every framework, so a custom prometheus_metrics_path is excluded too. To trace the metrics route, set prometheus_metrics_path="".

Tests

• FastAPI and FastMCP: with health spans off, /metrics and /health/ produce no span, while /api/metrics-report and /api/health/details do. FastMCP also checks /health without the slash. These tests failed before the change.
• FastAPI and FastMCP: a custom prometheus_metrics_path is excluded.
• Three existing FastMCP tests used /metrics as their "still traced" control, which relied on a custom list dropping the default; they now use /health/ or an unknown path.
• ruff format --check, ruff check and mypy . pass; just test: 346 passed. Untouched files have the same uncovered lines as before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	99.34% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
microbootstrap/bootstrappers/fastapi.py	100.00% <ø> (ø)
microbootstrap/bootstrappers/fastmcp.py	100.00% <ø> (ø)
...obootstrap/instruments/opentelemetry_instrument.py	100.00% <100.00%> (ø)

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