This is needed because the netbox container runs as the "unit" user rather than root and can't write to the directory. Docker hasn't added the commands to change the uid of the tmpfs mount, so you need to override the mode instead. See https://stackoverflow.com/questions/53498380/docker-compose-mounting-a-tmpfs-usable-by-non-root-user

Monitoring.md

@@ -14,6 +14,8 @@ services:
     - type: tmpfs
       target: /tmp/metrics
       read_only: false
+      tmpfs:
+        mode: 0o01777
 
   # nginx
   nginx-exporter: