prometheus-node-exporter-lua: Add wifi_station_count

To return the number of connected clients.

At present this can be partially inferred by using a count() over one of
the existing metrics, however this doesn't handle the case when there
are no connected clients. When that happens, the count() will return no
data instead of 0.

Signed-off-by: Alex Tomlins <alex@tomlins.org.uk>

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

@@ -2,6 +2,7 @@ local ubus = require "ubus"
 local iwinfo = require "iwinfo"
 
 local function scrape()
+  local metric_wifi_stations = metric("wifi_stations", "gauge")
   local metric_wifi_station_signal = metric("wifi_station_signal_dbm","gauge")
   local metric_wifi_station_tx_packets = metric("wifi_station_tx_packets_total","counter")
   local metric_wifi_station_rx_packets = metric("wifi_station_rx_packets_total","counter")
@@ -13,6 +14,7 @@ local function scrape()
     for _, intf in ipairs(dev_table['interfaces']) do
       local ifname = intf['ifname']
       local iw = iwinfo[iwinfo.type(ifname)]
+      local count = 0
 
       local assoclist = iw.assoclist(ifname)
       for mac, station in pairs(assoclist) do
@@ -23,7 +25,9 @@ local function scrape()
         metric_wifi_station_signal(labels, station.signal)
         metric_wifi_station_tx_packets(labels, station.tx_packets)
         metric_wifi_station_rx_packets(labels, station.rx_packets)
+        count = count + 1
       end
+      metric_wifi_stations({ifname = ifname}, count)
     end
   end
 end