Extract data from configuration file /etc/config/dhcp and create labels
{name, ip, mac, dns} via uci. Those labels are useful in order to craft
complex prometheus queries as replacing the MAC address to a custom
name. E.g.: wifi_station_signal_dbm * on (mac) group_left(name)
uci_dhcp_host or on (mac) label_replace(wifi_station_signal_dbm, "name",
"$1", "mac", "(.+)")
Signed-off-by: Gérondal Thibault <contact@tycale.be>