feat(opennebula): support ETHx_METRIC for default route priority

[mcanevet]

Proposed Commit Message

feat(opennebula): support ETHx_METRIC for default route priority

Add `get_metric()` to `OpenNebulaNetwork` and use it in `gen_conf()`.
When `ETHx_METRIC` is set, the default gateway is emitted as an explicit
Netplan v2 `routes:` entry with the metric attached, rather than the
simpler `gateway4:` key. When the variable is absent the existing
`gateway4:` behaviour is preserved.

This allows correct route priority on multi-homed VMs: setting
`ETH0_METRIC=0` and `ETH1_METRIC=1` ensures interface 0 is the
preferred default route.

Additional Context

The OpenNebula context-linux package (one-apps) supports ETHx_METRIC
to set the route metric for the default gateway on each interface.
Without this, multi-homed VMs (common in OpenNebula deployments using
tools like netbox-network-config) have no way to express route priority
and the kernel picks an arbitrary default route.

The fix is minimal: when ETHx_METRIC is present the gateway4: key is
replaced by an explicit routes: entry carrying the metric. When the
variable is absent behaviour is identical to before.

Test Steps

On a multi-homed OpenNebula VM, add to the VM template context:

ETH0_GATEWAY="10.0.0.1"
ETH0_METRIC="0"
ETH1_GATEWAY="185.70.43.1"
ETH1_METRIC="1"

After boot, verify route priorities:

ip route show
# expected: default via 10.0.0.1 dev eth0 metric 0
#           default via 185.70.43.1 dev eth1 metric 1

Also verify that a VM with no ETHx_METRIC set continues to produce
gateway4: in its Netplan config without any change in behaviour.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[github-actions]

Hello! Thank you for this proposed change to cloud-init. This pull request is now marked as stale as it has not seen any activity in 14 days. If no activity occurs within the next 7 days, this pull request will automatically close.

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[mcanevet]

@blackboxsw rebased

[Copilot]

Pull request overview

This PR enhances the OpenNebula datasource’s Netplan v2 generation to support per-interface default-route priority via ETH<x>_METRIC, aligning cloud-init behavior with OpenNebula’s context-linux contextualization variables for multi-homed VMs.

Changes:

• Add OpenNebulaNetwork.get_metric() and use it in gen_conf() to emit the IPv4 default gateway as an explicit routes: entry with metric when ETH<x>_METRIC is set (otherwise preserve gateway4: behavior).
• Add unit tests covering get_metric() and the gen_conf() metric-based route emission behavior.
• Document ETH<x>_METRIC in the OpenNebula datasource reference.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cloudinit/sources/DataSourceOpenNebula.py	Adds metric lookup and conditionally emits default route with a metric using Netplan routes:.
tests/unittests/sources/test_opennebula.py	Adds unit tests for ETH<x>_METRIC parsing and Netplan config output.
doc/rtd/reference/datasources/opennebula.rst	Documents the new ETH<x>_METRIC contextualization variable and its effect on default routing.

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