fix ceilometer notification settings in the nova.conf

[wilkmar]

This is a follow up to the previous commit [1], which did not properly configured nova.conf to support Ceilometer notifications.

On 2026.1, according to [2], all instance-related metric are collected by pollster, so theoretically, no nova.conf changes are needed at all. However, when enabled, Nova sends the legacy, unversioned notifications to Ceilometer, with the following meters, which are not available via pollster [4]:

• memory
• vcpus
• disk.root.size
• disk.ephemeral.size

Without the Nova notifications enabled the above, flavor-based, meters will not be available. This is important because some external or third party billing systems may rely on that.

This commit fixes missing nova.conf Ceilometer notification settings.

[1] #208
[2] https://docs.openstack.org/ceilometer/2026.1/admin/telemetry-measurements.html#openstack-compute
[3] https://opendev.org/openstack/nova/src/branch/stable/2026.1/nova/compute/utils.py#L342
[4] https://opendev.org/openstack/nova/src/branch/stable/2026.1/nova/notifications/base.py#L337

Partial-bug: #2162762

QA steps

1. Deploy sunbeam
2. Build and install the openstack-hypervisor snap with this branch
3. Enable telemetry

sunbeam enable telemetry

4. Verify nova-compute config

# in the /var/snap/openstack-hypervisor/common/etc/nova/nova.conf
[DEFAULT]
...
instance_usage_audit = True
instance_usage_audit_period = hour
...
[oslo_messaging_notifications]
driver = messagingv2

[notifications]
notify_on_state_change = vm_and_task_state
notification_format = both

5. Launch an instance and verify the vcpus metric

openstack server create --flavor m1.small --image ubuntu --network demo-network test-vm

# wait up to 1 hour for the notification to reach the ceilometer
openstack metric resource show <instance-id>
# example output below:
+-----------------------+---------------------------------------------------------------------+
| Field                 | Value                                                               |
+-----------------------+---------------------------------------------------------------------+
| id                    | e3c77d32-7ca1-4645-94ef-dad46a2bc8aa                                |
| creator               | 1a3a6b1187b74e9c924f08a5518b6435:9830ca69a9424df59ca7a826e8372ad3   |
| started_at            | 2026-09-01T14:31:05.712492+00:00                                    |
| revision_start        | 2026-09-01T14:33:14.888301+00:00                                    |
| ended_at              | None                                                                |
| user_id               | 1db99cff9d6042bcb513aaac19ecb751                                    |
| project_id            | 30e7e876c3b34363bbe2f9c00812fae6                                    |
| original_resource_id  | e3c77d32-7ca1-4645-94ef-dad46a2bc8aa                                |
| type                  | instance                                                            |
| revision_end          | None                                                                |
| metrics               | compute.instance.booting.time: 35878227-25ab-4115-9048-a93a08612af7 |
|                       | cpu: ad4a04a2-d390-4637-88a1-9b7265e71a0e                           |
|                       | disk.ephemeral.size: 58614ce8-14a7-426a-afd2-d07f443ffbbc           |
|                       | disk.root.size: f3600743-11da-4e1e-b858-7f1772f07332                |
|                       | memory.resident: b417f738-e055-4805-b95d-039558067a6e               |
|                       | memory.usage: 48f9dab2-2753-42ba-bfef-e07dbf470038                  |
|                       | memory: cbc0c6cd-e25b-4bf2-b0be-bd8f0c44e641                        |
|                       | vcpus: eb617089-959a-48c9-9358-a87c627067c9                         |
| created_by_user_id    | 1a3a6b1187b74e9c924f08a5518b6435                                    |
| created_by_project_id | 9830ca69a9424df59ca7a826e8372ad3                                    |
+-----------------------+---------------------------------------------------------------------+

Links

Jira card: OPEN-4670