fix(medcat-service): Fix Prometheus metrics and add unit tests for it (#221)

Author: alhendrickson

medcat-service/medcat_service/main.py

@@ -5,6 +5,7 @@
 from fastapi import FastAPI, Request
 from fastapi.responses import JSONResponse
 
+from medcat_service.config import Settings
 from medcat_service.demo.gradio_demo import io
 from medcat_service.dependencies import get_settings
 from medcat_service.log_config import log_config
@@ -36,10 +37,17 @@
 
 gr.mount_gradio_app(app, io, path="/demo")
 
-if settings.observability.enable_metrics:
-    from prometheus_fastapi_instrumentator import Instrumentator
 
-    Instrumentator(excluded_handlers=["/api/health.*", "/metrics"],).instrument(app).expose(app, tags=["admin"])
+def configure_observability(settings: Settings, app: FastAPI):
+    if settings.observability.enable_metrics:
+        from prometheus_fastapi_instrumentator import Instrumentator
+
+        Instrumentator(
+            excluded_handlers=["/api/health.*", "/metrics"],
+        ).instrument(app).expose(app, tags=["admin"])
+
+
+configure_observability(settings, app)
 
 
 @app.exception_handler(HealthCheckFailedException)

medcat-service/medcat_service/test/test_admin.py

@@ -1,13 +1,30 @@
+import sys
 import unittest
 
 from fastapi.testclient import TestClient
 
-from medcat_service.main import app
+from medcat_service.config import ObservabilitySettings, Settings
+from medcat_service.main import app, configure_observability
 from medcat_service.test.common import setup_medcat_processor
 
 
 class TestAdminApi(unittest.TestCase):
     ENDPOINT_INFO_ENDPOINT = "/api/info"
+    METRICS_ENDPOINT = "/metrics"
+
+    def _reload_app(self):
+        """
+        Reload the FastAPI app after env changes
+        Used to fix this error when trying to change the observability
+        "Cannot add middleware after an application has started"
+        """
+        # Clear cached imports so settings are re-evaluated
+        for mod in list(sys.modules):
+            if mod.startswith("medcat_service"):
+                sys.modules.pop(mod)
+        from medcat_service.main import app
+
+        return app
 
     def setUp(self):
         setup_medcat_processor()
@@ -17,6 +34,26 @@ def testGetInfo(self):
         response = self.client.get(self.ENDPOINT_INFO_ENDPOINT)
         self.assertEqual(response.status_code, 200)
 
+    def test_get_metrics_enabled(self):
+        settings = Settings(observability=ObservabilitySettings(enable_metrics=True))
+        app = self._reload_app()
+        configure_observability(settings, app)
+        client = TestClient(app)
+
+        response = client.get(self.METRICS_ENDPOINT)
+        self.assertEqual(response.status_code, 200)
+        self.maxDiff = None
+        self.assertTrue("http_requests_total" in response.text)
+
+    def test_get_metrics_disabled(self):
+        app = self._reload_app()
+        settings = Settings(observability=ObservabilitySettings(enable_metrics=False))
+        configure_observability(settings, app)
+        self.client = TestClient(app)
+
+        response = self.client.get(self.METRICS_ENDPOINT)
+        self.assertEqual(response.status_code, 404)
+
 
 if __name__ == "__main__":
     unittest.main()

medcat-service/requirements.txt

@@ -10,3 +10,4 @@ fastapi[standard]==0.115.2
 pydantic==2.9.2
 pydantic-settings==2.10.1
 gradio==5.38.0
+prometheus-fastapi-instrumentator==7.1.0

medcat-service/start_service_debug.sh

@@ -11,4 +11,6 @@ if [ -z "${APP_MODEL_CDB_PATH}" ] && [ -z "${APP_MODEL_VOCAB_PATH}" ] && [ -z "$
   echo "Using default model pack in  $APP_MEDCAT_MODEL_PACK"
 fi
 
+export APP_ENABLE_METRICS=${APP_ENABLE_METRICS:-True}
+
 fastapi dev medcat_service/main.py