How to get CPU usage in MySQL per user?
Some resource limitations: https://dev.mysql.com/doc/refman/5.6/en/user-resources.html and https://mariadb.com/kb/en/library/grant/#resource-limit-options
Switch to MariaDB to get "user stats" (connections, bytes, rows, CPU, etc): https://mariadb.com/kb/en/library/information-schema-user_statistics-table/ and  https://mariadb.com/kb/en/library/user-statistics/