fix(base): metrics dir env var PROMETHEUS_MULTIPROC_DIR must ALWAYS exist

if it don't exist python cant set it.

ref: #469 #470

.vscode/launch.json

@@ -35,7 +35,10 @@
             ],
             "django": true,
             "autoStartBrowser": false,
-            "cwd": "${workspaceFolder}/app"
+            "cwd": "${workspaceFolder}/app",
+            "env": {
+                "PROMETHEUS_MULTIPROC_DIR": ""
+            }
         },
         {
             "name": "Migrate",

dockerfile

@@ -100,6 +100,10 @@ ENV CI_PROJECT_URL=${CI_PROJECT_URL}
 ENV CI_COMMIT_SHA=${CI_COMMIT_SHA}
 ENV CI_COMMIT_TAG=${CI_COMMIT_TAG}
 
+# Var must exist, even empty so that the metrics settings logic functions
+# correctly
+ENV PROMETHEUS_MULTIPROC_DIR=""
+
 ENV IS_WORKER=False