By default, if no metric is defined, the kernel interprets the
highest value (0).
The current implementation, using pyroute2, is a translation from
the CLI command "ip route". This command uses the netlink API to
communicate with the kernel. In IPv6, when the metric value is not
set is translated as 1024 as default [1].
[1]https://access.redhat.com/solutions/3659171
Change-Id: I0c5f9e320bbbf314a2d6a22c515bf903de84cdaf
Related-Bug: #1855759