FortiGate's SNMP configuration lives under System > SNMP in FortiOS, with separate community and host-access settings.
From the CLI, create an SNMP community and scope which hosts are allowed to query it, rather than leaving it open to the whole management subnet.
config system snmp community
edit 1
set name "linivo"
set hosts "10.20.0.50/32"
next
end
SNMP querying only works once the agent itself is turned on for the interface it's reachable from, under System > SNMP > SNMP Agent in the GUI.
Fortinet's MIB reports interface counters, CPU/memory, session count and HA status; session count in particular is a useful early signal on a firewall approaching capacity.