Windows proxy metric registration was in a separate file, which had
led to some metrics (eg the new ProxyHealthzTotal and ProxyLivezTotal)
not being registered for Windows even though they were implemented by
platform-generic code.
(A few other metrics were neither registered on, nor implemented on
Windows, and that's probably a bug.)
Also, beyond linux-vs-windows, make it clearer which metrics are
specific to individual backends.