chore(mcp): remove unused observe_tool / observe_service decorators in favor of native fastmcp 3.x OTel spans

[dougborg]

Problem

frontapp_mcp_server/src/frontapp_mcp/logging.py:181,248 exports two decorators — observe_tool and observe_service — that are defined but unused anywhere in the tools surface (verified via grep -rln "observe_tool\|observe_service" frontapp_mcp_server/src/frontapp_mcp/tools/).

They mirror exactly the pattern stocktrim deleted in PR #157 (-731 LoC). Stocktrim found that fastmcp 3.x's native OTel span instrumentation covers everything the decorators did, while letting operators choose their own backend (Honeycomb, Datadog, Grafana, etc.) instead of being prescribed one.

Solution

After the fastmcp 3.x bump (#108):

1. Delete observe_tool and observe_service from logging.py (and any related test code)
2. Delete the __all__ exports
3. Add a short frontapp_mcp_server/docs/observability.md describing how operators wire OpenTelemetry into the server (env vars, exporter configuration, sample setup)

Depends on

• chore(mcp): migrate fastmcp 2.x → 3.x (precondition for caching middleware, prompts, prefab UI) #108 (fastmcp 3.x migration)

Files to change

• frontapp_mcp_server/src/frontapp_mcp/logging.py — remove decorators + exports
• frontapp_mcp_server/tests/ — delete any tests of the removed decorators
• frontapp_mcp_server/docs/observability.md (new)

Acceptance criteria

• No observe_tool / observe_service anywhere in frontapp_mcp_server/
• uv run poe full-check passes
• observability.md shows a working OTel export setup

Reference

• refactor(mcp): delete bespoke @observe_* decorators; document operator hooks stocktrim-openapi-client#157 — exact pattern (deleted observability.py + 11 decorator uses + tests)

[dougborg]

Cross-reference (2026-05-06)

When this lands, also delete frontapp_mcp_server/tests/test_observability_decorators.py. It's the only file in the test suite with intentionally timing-sensitive tests (real await asyncio.sleep(0.1) to verify the decorators measure duration). Both the decorators and the tests go away together.

Audit confirming these are the only timing-fragile tests in the codebase: see closure comment on #106.