The official node_exporter reports node_time_seconds as a gauge, but
prometheus-node-exporter-lua reports it as a counter. To be consistent
with the official implementation, and because "gauge" is more correct
than "counter" for this metric (system time can decrease, but the
Prometheus documentation states, "A counter is a cumulative metric that
represents a single monotonically increasing counter whose value can
only increase or be reset to zero on restart."), change the type for
node_time_seconds to "gauge".

Signed-off-by: Forest Crossman <cyrozap@gmail.com>