Added frontend-nlb-status-only annotation to fix ExternalDNS record creation when using a frontend NLB

[dominikhei]

Issue

#4805

The original issue was surfaced in external-dns#5661, where it was decided that the fix belongs in the AWS Load Balancer Controller rather than ExternalDNS

Description

When frontend NLB is configured via alb.ingress.kubernetes.io/enable-frontend-nlb: "true", the controller writes both the ALB and NLB hostnames to status.loadBalancer.ingress. When using ExternalDNS on AWS, only a DNS record for the first entry is created, meaning the NLB, which is the actual external-facing endpoint, may not get a DNS record at all.

The existing workaround (external-dns.alpha.kubernetes.io/target) requires a two-step deploy because the NLB hostname isn't known until after provisioning, which makes it impractical for automated workflows.

This PR adds a new boolean annotation:
alb.ingress.kubernetes.io/frontend-nlb-status-only: "true"

When set, only the NLB hostname is written to status.loadBalancer.ingress, so ExternalDNS creates a record for the NLB only. The annotation is opt-in to avoid a breaking change. If the NLB DNS is not yet available, the controller falls back to the ALB hostname to avoid leaving status empty.

Validation:

Manually validated on EKS: ingress with the annotation shows 1 status entry (NLB only) and a control ingress without it shows 2 entries (ALB + NLB).

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[kubernetes-prow]

Hi @dominikhei. Thanks for your PR.

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: dominikhei
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[zac-nixon]

/ok-to-test

[zac-nixon]

I wonder if we just want to block setting any hostname if you have this flag set, until the nlb is provisioned? that way there is no weird host flip / flop when you're provisioning an nlb -> alb stack.

[dominikhei]

The change would be pretty simple from a code perspective. I don't see any downside apart from the fact that if anything is waiting on ingress status to be populated before considering the ingress ready could be blocked for longer. But since the current behavior would populate it with the ALB hostname, they would be unblocking on the wrong address anyway...