I couldn't find a way to filter the attributes of the template neither. However, you can do something with metrics of the template by adding Report Limit.
If you use the template to create a report, you can apply a filter to the report.
If you use the template for drilling from an attribute of a report, MSTR automatically applies some filters to the template based on the parent report to create a child report.
That is probably why MSTR don't allow to add filter in template. Anyone corrects me if I am wrong.
tony