Optional
onCalled once for each created connection.
Optional
onCalled every time a connection is acquired from the connection pool.
A mysql2 Pool instance or a function that returns one.
If a function is provided, it's called once when the first query is executed.
https://github.com/sidorares/node-mysql2#using-connection-pools
Config for the MySQL dialect.
https://github.com/sidorares/node-mysql2#using-connection-pools