Add logfire sampling configuration and fix boolean env var handling

[zzstoatzz]

Logfire Sampling Configuration

This PR introduces configurable logfire sampling to reduce span budget consumption for high-traffic Prefect server deployments.

Configuration

All settings are optional with sane defaults:

PREFECT_LOGFIRE_SAMPLING_HEAD_RATE=0.1              # 10% of traces
PREFECT_LOGFIRE_SAMPLING_LEVEL_THRESHOLD=warn       # Always log warn+
PREFECT_LOGFIRE_SAMPLING_DURATION_THRESHOLD=5.0     # Always log >5s spans
PREFECT_LOGFIRE_SAMPLING_BACKGROUND_RATE=0.01       # 1% tail sampling

Implementation

• Created src/prefect/_internal/observability.py to centralize logfire configuration
• Replaced duplicated logfire setup in server.py and configurations.py
• Used Pydantic BaseSettings for configuration with environment variables
• Added TypeAdapter for proper boolean parsing (fixes PREFECT_LOGFIRE_ENABLED env var check is not properly handling false values #19360)

Boolean Environment Variable Fix

Fixes #19360 where PREFECT_LOGFIRE_ENABLED=0 doesn't properly disable logfire.

Previously, os.getenv("PREFECT_LOGFIRE_ENABLED") returned a truthy string for any non-empty value, including "0", "false", etc. Now uses Pydantic's TypeAdapter(bool) to properly parse boolean environment variables.

OpenAPI Schema Fix

Excludes /admin/settings endpoint from the OpenAPI schema to prevent Settings type pollution.

Problem

The /admin/settings endpoint returns the full prefect.settings.Settings object, causing FastAPI to include the entire Settings model tree (APISettings, CLISettings, ServerSettings, LoggingSettings, etc.) in the generated OpenAPI schema. This resulted in 2000+ lines of spurious Settings type definitions being added to ui-v2/src/api/prefect.ts every time someone modified a file matching src/prefect/server/api/*, polluting PRs with unrelated changes.

Solution

• Added include_in_schema=False to the /admin/settings endpoint (src/prefect/server/api/admin.py:15)
• Updated test_server_routes_match_openapi_schema to allow this exclusion
• The UI doesn't need these type definitions since it just displays settings as raw JSON

Verification

After this change, the pre-commit hook's OpenAPI sync generates a stable schema without the Settings types.

Test Plan

• uv run pytest tests/client/test_client_routes.py::test_server_routes_match_openapi_schema
• Verify logfire sampling configuration via environment variables
• Verify PREFECT_LOGFIRE_ENABLED=0 properly disables logfire
• Verify pre-commit hooks generate stable TypeScript schema

Closes #19360
Supersedes #19362

[codspeed-hq]

CodSpeed Performance Report

Merging #19372 will not alter performance

_{Comparing nate/logfire-sampling-and-bool-fix (a284483) with main (abcb29c)}

Summary

✅ 2 untouched