We were having trouble monitoring Fortigate switches once they had been brought under Fortimanager control as they no longer can be queried directly with SNMP. The switches get onboarded to a 169.x.x.x management network and while it might be possible to make firewall rules etc and use the Fortinet_FortiSwitch datasources it wouldnt be fun or practical in all collector deployments.
So we made an addition to fortigate
Fortinet_FortiGate_ManagedSwitch
Published with identifier MK3TRR
Collects;
1. switch status (up/down) with an alert
2. Status of ports 1-52 and collates into a total switch port utilisation complex datapoint with graph.
Hope this is helpful