Preserve backend ports and host rewrites in endpoint probes

[kahirokunn]

Problem

Before switching a KIngress to new Envoy programming, net-contour creates a temporary --ep KIngress that keeps the current and previous backends available while Envoy loads the new endpoint configuration.

For example, an update can replace this previously programmed HTTPProxy route:

routes:
- requestHeadersPolicy:
    set:
    - name: Host
      value: app.default.svc.cluster.local
  services:
  - name: app
    port: 80

with this KIngress backend:

splits:
- serviceName: app
  servicePort: 8080

The endpoint probe grouped backends only by Service name, so app:8080 replaced app:80 in the temporary route. When a previous Service was restored, its route-level Host rewrite was also lost. The probe could therefore omit the previous Envoy cluster or forward its synthetic *.net-contour.invalid host to a backend that requires the internal host, leaving the KIngress in EndpointsNotReady.

The Host rewrite case is exercised by knative/serving#16685, which changes a DomainMapping KIngress from a cluster-local Service backend to direct Revision Service backends.

Changes

Key endpoint-probe backends by Service name and port, include the port in each synthetic probe host, and restore the previous HTTPProxy route's Host rewrite.

Testing

Unit tests cover changed numeric ports, named ports, current-setting precedence for an unchanged backend, and Host rewrite recovery for previous backends.

/kind bug

Release Note

Fixed endpoint probes for previous backends that use a different Service port or require host rewriting.

Docs

N/A

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kahirokunn
Once this PR has been reviewed and has the lgtm label, please assign skonto for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.74%. Comparing base (577c757) to head (231a047).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1301      +/-   ##
==========================================
+ Coverage   93.41%   93.74%   +0.32%     
==========================================
  Files           9        9              
  Lines         653      687      +34     
==========================================
+ Hits          610      644      +34     
  Misses         22       22              
  Partials       21       21              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kahirokunn]

@linkvt Hi ✋ Could you please review this PR when you have time, if you don't mind? Thank you🙇