Use "\n" as delimiter as it cannot occur in the series name.

When config dir is mounted from configmap, filepath.Walk() finds the same
.conf file twice as 20-acme.conf is a link to ..data/20-acme.conf for example.

This patch skips all folder names starting with '..' which is pretty
uncommon and mainly used by Kubernetes mounts.

To match existing metric style.

This keeps the format of this file the same between systemd and
sysvinit.