route statements without metric are silently ignored

[George Shuklin]

If routes element have no metric key, it is silently ignored.

Example configuration:

network:
    version: 2
    renderer: networkd
    ethernets:
        ens7:
           dhcp4: true
           routes:
            - to: 192.168.1.0/24
              via: 10.160.0.1

Expected behavior:
- Warn user or reject config

Actual behavior:
Nothing in networkd configuration.