Thank you!! After reading and practicing with --test a few times I think the lightbulb has clicked. I just needed to add [inputs.snmp.tags] Its funny how everything is obvious in hindsight. just in case anyone has the same question in the future, this is what I learned:
testing the config in my first post (before adding the [global_tags] section) yielded the output below.
snmp,agent_host=******,host=***** UPS_A_kW=162 1630531173000000000
After adding the [global_tags] section, this was the output:
> snmp,agent_host=********,center=unknown,grid=unknown,host=houdcim02,ups=unknown UPS_A_kW=163 1630531556000000000
Notice how the global tags are now listed in the output as “center = unknown” etc.
The next step was to add [inputs.snmp.tags] and define them as below:
"center" = "USI2"
"ups" = "A"
Which yielded these results:
> snmp,agent_host=*******,center=USI2,grid=unknown,host=houdcim02,ups=A UPS_A_kW=162 1630531966000000000
Thank you for the help Anaisdg! I hope this can help some other beginner in the future.