This is very different question than your original question.
alias
is used to name and distinguish different plugins from each other. This is especially helpful for logging. It is not used to rename metrics or change the data when sending it.
I think the easiest method to add your “alias” to your metrics is to add a new tag for each snmp
output:
[[inputs.snmp]]
agents=["udp://11.12.21.21:161"]
tags = {alias = "UK-server"}
You could also use a processor to swap out the agent_host
for your alias if you didn’t want antoher tag.