Could you try
/usr/bin/snmpwalk -v2c -c COMMUNITY -Ovq -M /opt/librenms/mibs:/opt/librenms/mibs/fortinet udp:HOSTNAME:161 FORTINET-FORTIGATE-MIB::fgSysCpuUsage
(I removed the final 0 and changed to snmpwalk command, so you should get the CpuUsage of all CPUs known to SNMP …)