cloud-init on azure with dual stack needs to add route-metric for dhcp6 also

cloud-init on azure generates netplan yaml that includes dhcp4 with the dhcp4-overrides of route-metric, i.e.:

            dhcp4: true
            dhcp4-overrides:
                route-metric: 100

however netplan requires the same route-metric for dhcp4 and dhcp6 as networkd doesn't allow specifying separate metrics for 4 vs 6, so when azure is setup with dual-stack the netplan yaml is not valid for netplan, i.e.:

            dhcp4: true
            dhcp4-overrides:
                route-metric: 100
            dhcp6: true

which causes netplan to exit with error:

Stderr: ERROR: eth0: networkd requires that route-metric has the same value in both dhcp4_overrides and dhcp6_overrides