Encapsulates every custom OCR metric in an OcrMetrics frozen dataclass and
exposes a `build_metrics(registry)` factory. Production main.py binds against
the default REGISTRY; tests construct a fresh CollectorRegistry per case and
monkeypatch main.metrics, so counter values stay isolated between tests
(decision #3 on issue #652, Option A).
Refs #652
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Locks down AC1: prometheus-fastapi-instrumentator must keep auto-exposing
http_requests_total and http_request_duration_seconds for application
traffic, not just register the /metrics endpoint.
Refs #652
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Mount the instrumentator immediately after FastAPI app creation, excluding
/health and /metrics from request metrics to keep http_requests_total focused
on real application traffic.
Refs #652
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
