prometheus-node-exporter-lua: add snmp6 exporter

If you want statistics about IPv6 you can use snmpv6 exporter.
Currently, the "/proc/net/snmp6" is existing but all values are
just "0".

To use this plugin you have to set
  CONFIG_PROC_STRIPPED=n

I will find a way to enable the important ipv6 statistics by default.

Signed-off-by: Nick Hainke <vincent@systemli.org>

utils/prometheus-node-exporter-lua/files/usr/lib/lua/prometheus-collectors/snmp6.lua

@@ -0,0 +1,39 @@
+local ubus = require "ubus"
+
+local function get_devices() -- based on hostapd_stations.lua
+    local u = ubus.connect()
+    local status = u:call("network.device", "status", {})
+    local devices = {}
+
+    for dev, dev_table in pairs(status) do
+        table.insert(devices, dev)
+    end
+    return devices
+end
+
+local function get_metric(device)
+    local label = {
+        device = device
+      }
+      
+    if device == "all" then
+        for e in io.lines("/proc/net/snmp6") do
+            local snmp6 = space_split(e)
+            metric("snmp6_" .. snmp6[1], "counter", label, tonumber(snmp6[2]))
+        end
+    else
+        for e in io.lines("/proc/net/dev_snmp6/" .. device) do
+            local snmp6 = space_split(e)
+            metric("snmp6_" .. snmp6[1], "counter", label, tonumber(snmp6[2]))
+        end
+    end
+end
+
+local function scrape()
+    get_metric("all")
+    for _, devicename in ipairs(get_devices()) do
+        get_metric(devicename)
+    end
+end
+
+return { scrape = scrape }