netplan does not support ifmetric

Yes, I believe we want to allow users to specify the RouteMetric value.
Netplan auto generates this value, but keeps it the same for any interface, except wifi which is set higher.

I believe if we allowed setting something like:

network:
    version: 2
        ethernets:
            eth0:
               dhcp4: y
               metric: 101
            eth1:
               dhcp4: y
               metric: 201

Which would render a networkd config like:

[Match]
Name=eth0

[Network]
DHCP=ipv4

[DHCP]
UseMTU=true
RouteMetric=101

[Match]
Name=eth1

[Network]
DHCP=ipv4

[DHCP]
UseMTU=true
RouteMetric=201

yes, it is how it worked before on ifupdown

I have opened https://github.com/CanonicalLtd/netplan/pull/39 to support a 'metric' key as described by Ryan.

I scratch my head over and over again. This netplan is half-baked solution, lots of missing feature from ifupdown. The bugs has been 2 years, yet not fixed.

I saw the later version can handle 'route-metric', but that only for DHCP injected route. What we need is interface metric for subnet route. When a host has multiple NIC on the same subnet, very much likely we want to be able to control the priority within this NICs.

According to https://netplan.io/examples, netplan supports route-metric keyword and I use it:

"network":
  "ethernets":
    "lan":
      dhcp4: true
      dhcp4-overrides:
              route-metric: 10
      "match":
        "name": "e*"
  "renderer": "networkd"
  "version": 2

This works with netplan.io v0.98-0ubuntu1. Don't know, whether this or newer version is available from standard repositories, but you can add 'proposed'repo and will be able to get it.

you may very easily migrate away off netplan and forget it forever.
that's for default "renderer" value which is "networkd".
for NetworkManager you usually don't need netplan at all, initially.
remove it, and NetworkManager would not ever notice that nor complain about something missing.
that should go for end-user Ubuntu versions like Lubuntu, Xubuntu and GNOME3 Ubuntu itself.
in networkd case, just copy netplan-generated files from /run/systemd/network/ into /etc/systemd/network/ , do a "systemctl enable systemd-networkd" , remove everything resembling netplan from your dpkg paackage list, and restart your system. keep your local console connected to the system for the first 1 or 2 system restarts, as you might need to adjust your .network/.netdev config file names priority.
after next 2..3 successful reboots, no thought about installing netplan should visit your head ever.

ideally, you should be able to run any gui/desktop Ubuntu variation with only systemd-networkd, without any NetworkManager, connman or, God forbid, netplan.
I use excellent ArchLnux wiki pages for my inspiration, like https://wiki.archlinux.org/index.php/Systemd-networkd
As well as its documentation on freedesktop.org wiki