`ubus.call(sprintf('network.interface.%s', ifname), 'status', {})`
returns null if the interface `ifname` doesn't exists (yet).
For pppoe interfaces using `option ipv6 auto`, a virtual interface suffixed `_6`
is automatically created once the connection is established,
but until then it doesn't exists.
Fixes: 6423781254 ("mwan3: reimplement rpcd plugin using ucode")
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
On my "test" router (5 wans, 2 tracking ips per wan), before any rework,
prometheus-node-exporter-lua mwan3 average scraping time was 1230ms
(scraping only the interfaces), after optimizing the shell version,
average time was down to 485ms, with ucode we are now at 41ms.
Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
