prometheus-node-exporter-lua: Add optional mwan3 collector

prometheus-node-exporter-lua: Add optional mwan3 collector

Supports interface metrics exposed by mwan3. The performance is a
little slow compared to other collectors (~300ms) as the ubus call is
where most of the time is spent. Any future speedups are likely better
put into mwan3's rpcd binary.

Signed-off-by: Ryan Doyle <ryan@doylenet.net>
[rename metrics,bump version]
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
(cherry picked from commit c279efb760ca516db7cdaee32216344df47f2203)