feat: add Tag field to IngressRule

[kahirokunn]

Allow downstream ingress controllers to propagate traffic tag metadata (e.g. as labels on HTTPRoute/HTTPProxy), improving observability and enabling tag-based extension controllers.

Changes

When a Knative Service has traffic tags, tagged HTTPRoutes are generated but have no metadata indicating which tag they belong to. This makes it difficult for external tools to identify the traffic tag from HTTPRoute resources.

The root cause is that IngressRule — the intermediate representation between knative/serving and ingress controllers — does not carry the tag name. The tag is only encoded in the hostname, forcing downstream controllers to reverse-parse it with fragile template-based heuristics (e.g. TagOfHost() in net-gateway-api).

This PR adds a Tag string field to IngressRule so that the tag name is explicitly available to ingress controllers. The field is omitempty and +optional, making it fully backward compatible.

• 🎁 Add Tag field to IngressRule struct

Context

In knative-extensions/net-gateway-api#931, we attempted to add a tag label to HTTPRoute by reverse-parsing the hostname using the tag template. However, since the tag template format is highly configurable, this approach turned out to be fragile. We concluded that the upstream IngressRule should carry the tag name explicitly rather than relying on hostname heuristics.

/kind api-change

Release Note

Add `Tag` field to `IngressRule` to propagate traffic tag metadata to downstream ingress controllers, improving observability of generated resources such as HTTPRoute.

Docs

N/A

[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (965e7c3) to head (e6f4518).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   93.33%   93.33%           
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  Files          35       35           
  Lines        1005     1005           
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  Hits          938      938           
  Misses         53       53           
  Partials       14       14           

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[linkvt]

/lgtm

[kahirokunn]

Thanks for the review! I did run ./hack/update-codegen.sh before pushing.

Happy to wait for @dprotaso's review on the overall direction. The corresponding serving PR is knative/serving#16437 for reference.

[dprotaso]

Talking in slack - I'm wondering if it's simpler to have a separate KIngress resource for the route tag. Then if the annotations of these resources would propagate over the child resources automatically. I think this would then work for all net-* implementations (as long as they are propagating annotations)

An additional benefit of this split is that the ingress resource should remain 'stable' (unchanged) as we make other route updates.

/hold

[kahirokunn]

We will handle this through another method, so we are closing this ticket.
knative/serving#16439