wireguard: add metric to routes

wireguard: add metric to routes

maintainer: Felix Fietkau <nbd@nbd.name>

The metric field is not used when wireGuard routes are created.

See: https://github.com/openwrt/openwrt/issues/23199

This patch is aimed at adding the metric field in the config to the created routes for WireGuard if
the metric field is not empty and not 0

Compile tested: X86, Openwrt-main
Run tested: X86

Please have a look and consider implementing

Signed-off-by: Erik Conijn <egc112@msn.com>
Link: https://github.com/openwrt/openwrt/pull/23280
Signed-off-by: Robert Marko <robimarko@gmail.com>