prometheus-node-exporter-lua: lower case bssid label value in wifi exporter

To allow cross matching bssids between different exporters we need to
use the same case, as label matching is case senstive.

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

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

@@ -18,7 +18,7 @@ local function scrape()
         local labels = {
           channel = iw.channel(ifname),
           ssid = iw.ssid(ifname),
-          bssid = iw.bssid(ifname),
+          bssid = string.lower(iw.bssid(ifname)),
           mode = iw.mode(ifname),
           ifname = ifname,
           country = iw.country(ifname),