When you add a new measure you can pick from the list of commonly used aggregations, or you can go into the expression editor to manually type the expression - in there are two helpful checkboxes for DISTINCT and TOTAL.
Not all users wish to use the expression editor, and even for those who do use it, it does need additional time.
Given the COUNT(DISTINCT <field>) aggregation is so common, can you expand the list of sample aggregations to include "COUNT DISTINCT". Although possibly less useful, an extra drop-down value for "COUNT TOTAL" would add consistency.
I do not expect the extra list value(s) would be difficult to add to the UI properties dialogue ?
Hopefully they would make the user experience better, and also prompt the user to think "Mmmmmm. Do I need to qualify my count?" leading to better / faster insights.