Junos Space Network Monitoring (OpenNMS) can handle part “monitor and raise a notification if threshold is crossed”.
SNMP polls for ifInOctets/ifInOctets for specifx ifindex. These polls could be to the specific group of interfaces, not all of them.
OpenNMS Notifications supports “send SNMP Trap” or “send HTTP GET/POST” to specific IP/URL, where off-box script is located .
Then off-bos script can login to the box and apply the config.