What does this PR do?

This adds a new metric to the network system check which indicates whether a particular Linux network interface is up.
The metric data comes from the sysfs entry /sys/class/net/<iface>/carrier. Description from the docs:

Indicates the current physical link state of the interface.
Possible values are:

== =====================
0  physical link is down
1  physical link is up
== =====================

Motivation

We are looking for a similar metric which the Prometheus Node Exporter already has:
https://github.com/prometheus/node_exporter/blob/0c10545e8dfb983f4c013281c4904be27f645de2/collector/netclass_linux.go#L91-L96

# curl -s localhost:9100/metrics | grep node_network_up
# HELP node_network_up Value is 1 if operstate is 'up', 0 otherwise.
# TYPE node_network_up gauge
node_network_up{device="cilium_host"} 1
node_network_up{device="cilium_net"} 1
node_network_up{device="ens5"} 1
node_network_up{device="ens6"} 1
node_network_up{device="lo"} 0
node_network_up{device="lxc11a8da8d4613"} 1
node_network_up{device="lxc1a771ce4e0b9"} 1
node_network_up{device="lxcef367b6368ea"} 1
node_network_up{device="nodelocaldns"} 0

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged