Page url:
https://mms.mongodb.com/help/management/permissions/

If you don't want to get DB Stats the minimum required role is only:

use admin
db.addUser( { user: "agent",
              pwd: "Moon1234",
              roles: [ "clusterAdmin"
                     ]
             } )

You can disable DBStats from the MMS settings page:

Settings -> General Settings and disable "DB Stats".