In the documentation for this add-on it states :
For assistance choosing metrics, use the Splunk Web configuration UI for this add-on to search and browse for the metrics and determine which ones require additional parameters. For a more information on the Citrix NetScaler NITRO API, refer to the NITRO API documentation.
I can't see any option in the Splunk Wed UI to browse the metrics ?
Also the Nitro API documentation link goes to an obsolete page.
For anyone else that finds this question, a good link for the Nitro metrics API is this: https://developer-docs.citrix.com/projects/netscaler-nitro-api/en/12.0/statistics/statistics/
OK i found the Metric browser in the Add Template
section 🙂