Add scalability_info field to ClusterState

[areina]

Add a field that indicates whether the cluster can be scaled up or down.

Instead of using a boolean field, which isn't extensible, we're adding a new message to represent information about the scalability of the cluster. At this moment the message contains the status and the reason for that status.

[areina]

@Rendez I think you were the original author of the /:cluster_id/scalable endpoint. For the new API, we are considering adding it as a property of the cluster state instead of as a method. We would like to get your point of view on this and also understand if it could have any downside for the existing use cases.

Additionally, we would prefer to take a more extensible approach, adding a ClusterScalability message instead of using a boolean field. That would allow us to add more information regarding the scaling options for a cluster. For instance, with scalable it isn't clear to me if that means whether the cluster can be scaled up or down. Is there currently more logic in the UI about the scaling of a cluster that could be offered by the API?

[github-actions]

The latest Buf updates on your PR. Results from workflow Pull Request / linting (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 29, 2025, 2:24 PM

[areina]

I had a discussion with @Rendez the other day, and the main reason to have a specific endpoint to retrieve this information was that the existing endpoint used to retrieve the cluster is expensive to compute and includes more fields than the necessary. I believe we can address both of these issues in the new api, and it's better to include this information as a field of the cluster rather than expose it through a separate method. I've updated the PR accordingly.

[Robert-Stam]

I don't see how we can e.g.

• make it possible to scale up, but not down
• make it possible to scale up to e.g. 16 nodes
• to scale down a 10 cluster to 8 (not smaller).

[Robert-Stam]

LGTM