Introduce a new flag that can be used to expose each DHCP lease as a
specific metric. Given the high cardinality of these metrics, exposition
of lease information is disabled by default.
This feature takes cues from node_exporter collectors which are
disabled by default, like the `processes` or `systemd` collectors for
instance. Typically, high-cardinality metrics are against prometheus
best-practices but for smaller networks (like home networks) these
features can be quite powerful.
