testing/prometheus-wireguard-exporter: new aport

There is a newer version, but it does not appear to build with Alpine's version of Rust. I'm not convinced this package is a good idea, but until I have time to rewrite it in something more sane, this is what I've got.

This technically depends on having 'wg' in path, but I wasn't sure if that should be expressed as a package manager dependency.