Add support for single mysqld_exporter collect multiple MySQL instances

[starsliao]

Add support for single mysqld_exporter collect multiple MySQL instances

• Example：

curl 'http://localhost:9104/probe?dsn=mysql_user1:password1@tcp(196.168.1.1:3306)/'
curl 'http://localhost:9104/probe?dsn=mysql_user2:password2@tcp(196.168.2.2:3306)/'

[SuperQ]

Supplying credentials in the URL params is not allowed as this is a security vulnerability.

Please take a look at prometheus-community/postgres_exporter#618 as a pattern for which this can be implemented.