default metric namespace and namespace from env

Author: vsergiienko

django_prometheus/conf/__init__.py

@@ -1,6 +1,12 @@
+import os
 from django.conf import settings
 
-if not settings.configured:
-    settings.configure()
+NAMESPACE = ""
 
-NAMESPACE = getattr(settings, "PROMETHEUS_METRIC_NAMESPACE", "")
+if settings.configured:
+    try:
+        NAMESPACE = settings.PROMETHEUS_METRIC_NAMESPACE
+    except AttributeError:
+        pass
+
+NAMESPACE = os.getenv("PROMETHEUS_METRIC_NAMESPACE", NAMESPACE)