KRT: Add kernel metric protocol option

KRT: Add kernel metric protocol option

Kernel routes with different metrics do not clash with each other,
therefore using dedicated metric value is a reliable way to avoid
overwriting routes from other sources (e.g. kernel device routes).

Although kernel route metric could already be set as a route attribute by
filters, that is not consistent with the way how Linux kernel handles
route metric - not just a route attribute, but a part of a route key.