Support updating runner scale-set labels in place (avoid delete-and-recreate)

[cbartz]

🏷️ Discussion Type

Product Feedback

💬 Feature/Topic Area

ARC (Actions Runner Controller)

Discussion Details

Summary

Runner scale-set labels are immutable after creation. To change the labels
a scale set advertises, you must delete the scale set and create a new one,
which forces a drain of any runners it owns and a window where matching jobs
queue. I'd like in-place label updates for runner scale sets.

Current behaviour

PATCH _apis/runtime/runnerscalesets/{id} with a changed labels array returns
200 OK but silently discards the change (GET shows the original labels).
So labels are effectively immutable, and the response doesn't signal that.
(Filed the silent-200 part as a correctness bug separately — link below.)

What I'd like

• labels editable via PATCH on an existing scale set, applied to subsequent
  job routing — without recreating the scale set or draining active runners.

Why

Declarative tooling (Kubernetes operators, GARM-based controllers, etc.)
reconciles desired config to live scale sets. A label change is a common,
benign config edit, but today it can only be realised as a destructive
recreate. In-place updates would make relabel a non-disruptive operation.

Workaround today

Delete-first recreation: drain → delete the old scale set → create a new one
with the same name and the new labels. Works, but disruptive.

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