prometheus-node-exporter-lua: fix corner case in hostapd_stations

There was a corner case, when a vif had no stations, that
evaluate_metrics for a station that was nil and had no collected metrics
would have been called.

Comment the code, to make it easier to understand and follow, and
simplify some variable names along the way.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>

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

@@ -139,26 +139,33 @@ local function scrape()
     local all_sta = handle:read("*a")
     handle:close()
 
-    local current_station = nil
-    local current_station_values = {}
+    local station = nil
+    local metrics = {}
 
     for line in all_sta:gmatch("[^\r\n]+") do
       if string.match(line, "^%x[0123456789aAbBcCdDeE]:%x%x:%x%x:%x%x:%x%x:%x%x$") then
-        if current_station ~= nil then
-          labels.station = current_station
-          evaluate_metrics(labels, current_station_values)
+        -- the first time we see a mac we have no previous station to eval, so don't
+        if station ~= nil then
+          labels.station = station
+          evaluate_metrics(labels, metrics)
         end
-        current_station = line
-        current_station_values = {}
+
+        -- remember current station bssid and truncate metrics
+        station = line
+        metrics = {}
       else
-        local name, value = string.match(line, "(.+)=(.+)")
-        if name ~= nil then
-          current_station_values[name] = value
+        local key, value = string.match(line, "(.+)=(.+)")
+        if key ~= nil then
+          metrics[key] = value
         end
       end
     end
-    labels.station = current_station
-    evaluate_metrics(labels, current_station_values)
+
+    -- the final station, check if there ever was one, will need a metrics eval as well
+    if station ~= nil then
+      labels.station = station
+      evaluate_metrics(labels, metrics)
+    end
   end
 end