Emit Prometheus 0.0.4 format version in /metrics Content-Type

kiosk_show_replacement/metrics.py

@@ -23,6 +23,13 @@
 
 metrics_bp = Blueprint("metrics", __name__)
 
+# Content type for the Prometheus text exposition format, version 0.0.4.
+# The ``version`` parameter is required: Prometheus v3 fails a scrape whose
+# Content-Type carries no recognized format version, where v2 silently fell
+# back to this format. This is the same value as prometheus_client's
+# ``CONTENT_TYPE_LATEST``.
+PROMETHEUS_CONTENT_TYPE = "text/plain; version=0.0.4; charset=utf-8"
+
 
 class MetricsCollector:
     """Thread-safe metrics collector for Prometheus-style metrics."""
@@ -504,7 +511,9 @@ def metrics_endpoint() -> Response:
     # Add summary metrics
     output += "\n" + get_summary_metrics()
 
-    return Response(output, mimetype="text/plain; charset=utf-8")
+    # Use content_type rather than mimetype: Werkzeug appends its own
+    # charset to a mimetype, which would duplicate the parameter here.
+    return Response(output, content_type=PROMETHEUS_CONTENT_TYPE)
 
 
 # Convenience functions for recording metrics from other modules

tests/unit/test_metrics.py

@@ -8,6 +8,7 @@
 """
 
 from kiosk_show_replacement.metrics import (
+    PROMETHEUS_CONTENT_TYPE,
     MetricsCollector,
     metrics_collector,
     record_database_error,
@@ -28,6 +29,18 @@ def test_metrics_endpoint_returns_text_plain(self, app, client):
         response = client.get("/metrics")
         assert "text/plain" in response.content_type
 
+    def test_metrics_endpoint_declares_exposition_format_version(self, app, client):
+        """Test the Content-Type names the Prometheus 0.0.4 text format.
+
+        Prometheus v3 fails a scrape whose Content-Type carries no recognized
+        format version, so the ``version`` parameter must be present, and the
+        header must not repeat ``charset``.
+        """
+        response = client.get("/metrics")
+
+        assert response.content_type == "text/plain; version=0.0.4; charset=utf-8"
+        assert response.content_type == PROMETHEUS_CONTENT_TYPE
+
     def test_metrics_contains_http_requests_help(self, app, client):
         """Test metrics output contains HTTP requests metric definition."""
         response = client.get("/metrics")