It's not needed. Whenever we add a route, we pass in the
route metric (for example, based on the device's configuration).
No need to merge and track it into the NMIP4Config/NMIP6Config.

The metric was only used in nm_ip4_config_create_setting()
and nm_ip6_config_create_setting(). In fact it's wrong to do
that, because it means we first capture some settings, and guess
the configured route metric. But we cannot do that. Our best
guess what a configured setting might be is -1.