zabbix server has a frontend script that executes

sudo /sbin/reboot

agent 2 has a config :

AllowKey=system.run[sudo /sbin/reboot]
DenyKey=system.run[*]

the agent will fail to reboot

solution 

AllowKey=system.run[sudo /sbin/reboot,wait]
DenyKey=system.run[*]

so the agent2 is expecting the 2nd parameter instead of using the default value

2020/06/16 12:31:10.109972 [1] registering new client
2020/06/16 12:31:10.110122 [1] cannot monitor metric "system.run[sudo /sbin/reboot,wait]": Unknown metric system.run