You would create a Software License with a metric of "Custom Metric". On the Identification tab of the license, you would set the metric to Gigabyte from the drop-down list. Next, you need to find out the current storage consumption from Splunk in GB. I assume there is some type of Admin Portal that provides this information.
Once you know the current amount of GB Storage from Splunk, open up your Splunk license and on the Consumption tab of the license, manually enter the GB Consumption into the Consumed Entitlement field.
During the next License Reconciliation, FNMS/ITAM will compare the number of GB entitlements that you have "purchased" against the amount of GB storage being consumed, and then set the appropriate compliance status of the license.
On a periodic based, you would need to update the actual GB Storage consumption on your license to make sure that this is kept up to date.
