Enable different backend monitoring configuration per hostgroup.

[rialg]

The mysql backend monitoring relies on sharing the same global variables for setting up hostgroup monitoring. This has the immediate effect that all backends must share the same monitoring user (monitor_username and monitor_password) , thus if this credentials don't exist in the mysql server, the heartbeats/ping will fails and potentially already working connections from other valid mysql_users will be killed by the hostgroup manager.

Incidentally, I am experiencing this issue when sharing the same config between unix socket connections that are authenticated using username:password credentials and IAM authenticated connections to mysql. Although an immediate workaround is to have two proxysql services running with different configs, it would make more sense for my set up to have all the hostgroups configured using the same instance.