Depends

To me this is where SNMP traps shine, its quite specifically its purpose. Which means alloy, logstash, snmptrapd + promtail, logstashetc. Which fits best depends on what youre doing, I personally lean simple first. Do you want traps by themselves? Remote logs? Do you have a device thats already handling relays that can be triggered? How does the UPS connect? If its serial or USB over to a dedicated machine, that machine should log and send upstream.