Add note about METRICS_ENABLED in env/netbox.env

Monitoring.md

@@ -1,5 +1,10 @@
 ## Setup
 
+### Note about environment variables
+
+If you use the below config to set "METRICS_ENABLED" to 'true' inside of docker-compose.override.yml, you will need to edit env/netbox.env and comment out the METRICS_ENABLED line there. This is (I think) because the environment file is sourced after the docker-compose override. I think this line should probably be removed from env/netbox.env to reduce confusion, since it's the default.
+
+
 Add the following to your `docker-compose.override.yml` (or create that file if you haven't done so):
 
 ```yml