prometheus-node-exporter-lua: extend bmx7 plugin

prometheus-node-exporter-lua: extend bmx7 plugin

Now monitors as well the tunIn parameters, aka the annoucend routes to
the network. This is handy to check if a node annouces itself as a
gateway.

Also list all activated plugins.

Example output:

    # TYPE bmx7_tunIn gauge
    bmx7_tunIn{name="myIP4",network="10.198.52.86/32"} 1
    bmx7_tunIn{name="myIP6",network="2012:0:0:56::/128"} 1
    bmx7_tunIn{name="inet4",network="0.0.0.0/0"} 1
    # TYPE bmx7_plugin gauge
    bmx7_plugin{name="bmx7_config.so"} 1
    bmx7_plugin{name="bmx7_json.so"} 1
    bmx7_plugin{name="bmx7_sms.so"} 1
    bmx7_plugin{name="bmx7_tun.so"} 1

Signed-off-by: Paul Spooren <redacted>