The issue with what you want is that if the FGT goes down, it's brain is shut off and it cannot alert anything. You need a monitoring tool that is located on the far side of the network because if it is behind the Fortigate, it may be susceptible to the same ISP or power issues the Fortigate is and you'll get nothing. Just my two cents. I run a Cacti server and monitor most of my devices as well as a friend's. That friend in turn monitors some of mine along with my Fortigate so there is redundancy if either of our Fortigates go south.
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