make generated metric exposition test more exhaustive

{{ cookiecutter.project_slug }}/src/{{ cookiecutter.project_slug }}/metrics/tests/test_metric_exposition.py

@@ -1,7 +1,12 @@
+import pytest
 from django.shortcuts import resolve_url
 from django.test import Client
+from prometheus_client.parser import text_string_to_metric_families
 
 
-def test_metric_endpoint_returns_200(client: Client):
+@pytest.mark.django_db
+def test_metric_endpoint_returns_valid_prometheus_data(client: Client):
     response = client.get(resolve_url("prometheus_metrics"))
     assert response.status_code == 200
+    metrics = list(text_string_to_metric_families(response.content.decode("UTF-8")))
+    assert len(metrics) > 0