What is the proper way to assign the privileges when a MySQL instance has multi dba?
Mysql offers different ways of creating user .

For application A you can create a user with specific required roles and its better to give IP from respective DBA operates instead of giving % which allows user to enter in mysql from anywhere. With this user you can check performance of your queries and also explain part.
So you can use :
Grant select, insert, update,delete to user@IP on db_a.* Identified​ by 'password';


For subset user you need to declare which admin rights you want to grant like version and all. Roles will depend on your requirement.


User can be created with same command as above.
Let me know if more clarity is needed