feat(ocr): expose /metrics endpoint via prometheus-fastapi-instrumentator

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>

ocr-service/test_metrics.py

@@ -0,0 +1,24 @@
+"""Tests for Prometheus metrics exposed by the OCR service.
+
+Each test that asserts on a counter/gauge value uses a fresh CollectorRegistry
+(see decision #3 on issue #652) to keep the metrics isolated between tests.
+"""
+
+from unittest.mock import patch
+
+import pytest
+from httpx import ASGITransport, AsyncClient
+
+from main import app
+
+
+@pytest.mark.asyncio
+async def test_metrics_endpoint_returns_200():
+    """`GET /metrics` returns 200 with Prometheus exposition content."""
+    with patch("main.kraken_engine.load_models"), \
+         patch("main.load_spell_checker"):
+        async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as client:
+            response = await client.get("/metrics")
+
+    assert response.status_code == 200
+    assert "text/plain" in response.headers.get("content-type", "")