Fortigate Rules

I haven't used Prometheus nor SNMP traps to the internet, nor FW policies to control it. But here's how I'd diagnose it.

Your first policy hasn't been hit (it shows 0B traffic), so there's no SNMP or "prom" traffic coming back in to the network from grafana.squiggle.com. (see what I did there? :) ) That possibly means you're missing a VIP on the external side, if you expect the external server to connect to something in your environment (instead of the internal device send data and expect a reply. We also can't see, from your picture, what the policy applies to, to verify that this is a WAN->LAN policy.  If you're expecting this traffic, you may well have to work on that first. If you were putting that in to cater for responses from the internal devices sending data to the external site, you don't need it, already established sessions that are permitted by an existing outbound policy don't need a separate policy. 

Your policy #3 shows that it is forwarding traffic (9.19GB of it so far). You're allowing ALL traffic irrespective of port/protocol, not sure if that was what you wanted to do or if that's just to try to check what is going on. Next I would capture some packets on the FGT from your source to your destination, and see what it looks like - based on the policy and the existing traffic flow, it looks like the data should get to the endpoint it needs; a capture will show you both sides of the traffic and make sure you're getting what you need as responses.

Is there a guide for setting up a firewall that Prometheus have given you, or that you're following?