TrueNAS has a built-in SNMP service under its System Settings, reporting ZFS pool and disk health once turned on.
In System Settings > Services, find SNMP and click the configure (pencil) icon, then toggle the service on.
Set a non-default community string, and enable "Start Automatically" so SNMP survives a reboot instead of needing to be manually re-enabled.
TrueNAS's MIB, layered on Net-SNMP, reports ZFS pool status and capacity, individual disk SMART data, and system load; a pool moving from ONLINE to DEGRADED shows up as a detection, not something you find during a routine check.