Add well-known labels and annotation for weighted waypoint canary

[ymesika]

What this does

Registers the well-known labels and annotation for the weighted waypoint canary feature (Design Doc), which lets a service shift a configurable percentage of its traffic to a second "canary" waypoint alongside its primary one. This is the migration primitive for moving a service between two waypoints (for example two waypoint revisions, or two waypoint implementations) without changing clients or introducing a second Service.

Three additions:

Name	Kind	Purpose
istio.io/use-waypoint-canary	label	Names the second (canary) waypoint, alongside the primary istio.io/use-waypoint.
istio.io/use-waypoint-canary-namespace	label	Cross-namespace reference for the canary waypoint (mirrors istio.io/use-waypoint-namespace).
istio.io/use-waypoint-canary-weight	annotation	Percentage (0-100) of the service's traffic shifted to the canary. Defaults to 0; invalid values keep the service on the primary.

These apply to Service, ServiceEntry, and Namespace - the service-level scope. We deliberately scope alpha narrower than istio.io/use-waypoint (which also covers Pod/WorkloadEntry): a pod-scoped canary is logically valid for in-mesh traffic, but it would need a separate weighted field on the Workload API and ztunnel's workload path, and it can't apply to ingress traffic (which is keyed off the service). Keeping to service-level scope for alpha keeps the change minimal and behaves identically across in-mesh and ingress. Workload scope can follow later.

Example:

metadata:
  labels:
    istio.io/use-waypoint: waypoint-a            # primary (existing, unchanged)
    istio.io/use-waypoint-canary: waypoint-b     # canary waypoint (Gateway name)
    # optional, only for cross-namespace canaries:
    # istio.io/use-waypoint-canary-namespace: waypoint-b-ns
  annotations:
    istio.io/use-waypoint-canary-weight: "5"     # 0..100, percent routed to the canary

Scope

This PR only adds the API surface: the well-known label/annotation definitions. The implementation lands separately.

Why the canary references are labels but the weight is an annotation

The canary name references (use-waypoint-canary and use-waypoint-canary-namespace) are labels, following their use-waypoint and use-waypoint-namespace siblings.
They name a waypoint the same way the primary reference does, so they live in the same space for consistency.

The weight is an annotation because it is a configuration value, not identity:

• Labels are for identifying and selecting objects. They are indexed and used in selectors. A traffic percentage is never something you would select on (kubectl get -l weight=50 is meaningless), so it does not belong in the label
  space. This is the standard Kubernetes label-vs-annotation split.
• Value-carrying config in this repo is already expressed as annotations, for example sidecar.istio.io/proxyCPULimit, sidecar.istio.io/proxyMemoryLimit, traffic.sidecar.istio.io/excludeInboundPorts, and traffic.sidecar.istio.io/includeOutboundPorts. All are numeric or list config, all are annotations.

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