prometheus-node-exporter-lua: Change node_time_seconds type to "gauge"

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>

utils/prometheus-node-exporter-lua/files/usr/lib/lua/prometheus-collectors/time.lua

@@ -1,6 +1,6 @@
 local function scrape()
   -- current time
-  metric("node_time_seconds", "counter", nil, os.time())
+  metric("node_time_seconds", "gauge", nil, os.time())
 end
 
 return { scrape = scrape }