Hi Guneet,
I found Palo Alto Firewall MIB files here, and downloaded Enterprise SNMP MIB 7.1, compiled PAN-COMMON-MIB using MIB Tools and found panLcDiskUsage OID. Is this disk usage metric you want to monitor? See below.
If it is so then you can deploy the SNMP MIB 7.1 on the device, compile PAN-COMMON-MIB using MIB Tools, map pacLcDiskUsage to an attribute and utilize SpectroWATCH to monitor the attribute.
Regards,
Widjaja.