Custom Inventory - Populate field for VMs from the VMware Notes field on ESXi/vCenter inventory.
You will need to Agentless inventory your ESXi server, and then see if any piece of useful information is being pulled via SNMP.
check these links:
VMWare ESXi Agentless
Permission Requirements for VMware Agentless Inventory
a good example:
Getting Hardware service time from ESXi hosts