Problems with ingress class support in Contour HTTPProxies

[nnz1024]

Describe the bug

Hi everyone! Glad to see this great project returns to life!

I have Contour with --ingress-class-name=contour, so it ignores any HTTPProxies that specify a different ingress class or omit it.

Flagger has initial support for setting Contour ingress class via annotation projectcontour.io/ingress.class.

First, it looks like a bug that Flagger removes this annotation after first canary step, I think, because it is missing in newMetadata.Annotations, so this results in a non-zero diff and update of annotations map.

Second, it might be more appropriate to use ingressClassName field of HTTPProxySpec to set ingress class for HTTPProxy. Already have patched Flagger working in my cluster without issues, so PR won't be a problem, if you guys will agree on that.

To Reproduce

1. Install Contour with --ingress-class-name=contour.
2. Install Flagger with -ingress-class=contour.
3. Deploy a Canary and parent HTTPProxy according with tutorial.
4. After first canary step, child HTTPProxy (controlled by Flagger) loses class annotation, so parent proxy becomes invalid.

Expected behavior

Flagger-contolled HTTPProxy keeps its ingress class.

Additional context

• Flagger version: 1.41.0
• Kubernetes version: 1.34.1
• Service Mesh provider: None
• Ingress provider: Contour 1.33