Otherwise, `postfix_up{path="/var/lib/postfix/queue/public/showq"}` will
always be `0` indicating an postfix outage because this is a unix domain
socket that cannot be connected to:
2021/12/03 14:50:46 Failed to scrape showq socket: dial unix /var/lib/postfix/queue/public/showq: socket: address family not supported by protocol
The postfix exporter needs to access postfix's `queue/public/` directory
to read the `showq` socket inside. Instead of making the public
directory world accessible, this sets the postfix exporter's group to
`postdrop` by default, when the postfix service is enabled.
