According to wikipedia:

... class of service (CoS) is a 3-bit field called the Priority Code Point (PCP) within an Ethernet frame header when using VLAN tagged frames as defined by IEEE 802.1Q. It specifies a priority value of between 0 and 7 inclusive that can be used by QoS disciplines to differentiate traffic.

I have an internet-facing linux router that i needed to configure this on; and while i see some content for various consumer firewalls, i havent seen much on how to configure this on linux.

With a vlan device vlan_enp4s0 configured using systemd-networkd:

# cat /etc/systemd/network/02_vlan_dhcp_client_hw.network
[Match]
Name=enp4s0

[Network]
VLAN=vlan_enp4s0

[Link]
ActivationPolicy=always-up
RequiredForOnline=no

# cat /etc/systemd/network/02_vlan_dhcp_client.netdev
[NetDev]
Name=vlan_enp4s0
Kind=vlan

[VLAN]
Id=2

one can then set the egress CoS:

vconfig set_egress_map vlan_enp4s0 0 3

A good way to run something like this on boot in modern linux is with networkd-dispatcher:

apt install networkd-dispatcher
systemctl enable networkd-dispatcher
echo "vconfig set_egress_map vlan_enp4s0 0 3" > /etc/networkd-dispatcher/routable.d/egress_cos.sh