In description to builtin template called "Template DB MySQL by Zabbix agent 2" says:
"2. Set in the {$MYSQL.DSN} macro the system data source name of the MySQL instance such as <protocol(host:port or /path/to/socket)/>."
But its not working anyway. Digging in GO part of source, im found that mysql.dsn shold by like that: "Default value: tcp://localhost:3306..
Requirements:..

• Must match the URI format.
• Supported sockets: TCP, Unix.

Examples:

• tcp://myhost
• unix:/var/run/mysql.sock "
  Please fix description. Its take a whole day to figure out right format.