added comment

restic/.env.template

@@ -32,4 +32,6 @@ CRONTAB=30 22  *  *  * /root/restic/run.sh 0 2>&1
 #        |_______________Minute (0-59)
 
 # Prometheus Gateways URL
+# no metrics will be sent if empty
+# issue: hostname should be appended to the URL in docker-compose.yml
 PROM_GW_URL=http://pushgateway:9091/metrics/job/restic/host/zino2