feat(server): migrate EventList to google.protobuf.Struct for K8s 1.3.5 compatibility (#25767)

[chansuke]

Closes #25767

Change the return type of ListResourceEvents and ListEvents gRPC APIs from k8s.io.api.core.v1.EventList to google.protobuf.Struct to avoid protobuf compatibility issues with Kubernetes 1.35+.

Affected endpoints

• application.ListResourceEvents
• applicationset.ListResourceEvents
• project.ListEvents

Breaking Changes

• gRPC clients must handle google.protobuf.Struct instead of EventList.
• REST API responses remain unchanged.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
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[ppapapetrou76]

I have a few questions/comments before I dive into more details

1. Why the implementation is not following the suggestion with the wrapped struct here
2. from a design point of view having GRPC and Rest returning different structs for the same call is not ideal
3. Following question 1 the code now is moving away from a strongly-typed EventList to an unstructured google.protobuf.Struct. This removes compile-time type safety and additionally the grpc clients now need to parse the returned struct, know/assume the expected structure...
4. I'm also skeptical about performance especially if the event list is quite big because the code now is taking several steps to transform it.

[chansuke]

Thank you for your comment.

1. As noted in upgrade to K8s 1.35 (K8s Go client 0.35) #25767 (comment), wrapping K8s types directly doesn't work due to protobuf library incompatibility (gogo/protobuf vs google.golang.org/protobuf). Following the Istio approach, I used google.protobuf.Struct directly to avoid this issue.
2. gRPC and REST already return the same content and grpc-gateway converts google.protobuf.Struct to JSON automatically. A wrapped struct would make the gRPC API more explicit but doesn't change the underlying data.
3. I can wrap the Struct in a dedicated message type (e.g., message ResourceEventsResponse { google.protobuf.Struct events = 1; }). This won't restore compile-time type safety, but it gives gRPC clients a known　message type rather than a raw Struct.
4. I'll use protojson.Unmarshal to eliminate the intermediate map[string]interface{} step.

[reggie-k]

Thanks for the PR!
We should be minded to how we manage the breaking change this introduces, attaching a PR that dealt with a different breaking change in the past, it may be relevant as a strategy:
#23695

[chansuke]

@reggie-k
Thank you for the reference! I looked into #23695 and the V2 endpoint approach makes sense for maintaining backwards compatibility.
However, the current EventList return type will break anyway when upgrading to K8s 1.35 so, I'm considering the below approach:

1. V2 endpoint: Add a new endpoint returning Struct, deprecate the old one, remove it in a future releease
2. Current approach: Change the existing endpoint with clear documentation in the upgrade guide

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.85%. Comparing base (e8539be) to head (9f0a861).
⚠️ Report is 52 commits behind head on master.

Files with missing lines	Patch %	Lines
server/events/events.go	69.23%	2 Missing and 2 partials ⚠️
server/project/project.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26322      +/-   ##
==========================================
+ Coverage   62.65%   62.85%   +0.20%     
==========================================
  Files         412      413       +1     
  Lines       55562    55651      +89     
==========================================
+ Hits        34814    34982     +168     
+ Misses      17426    17336      -90     
- Partials     3322     3333      +11     

☔ View full report in Codecov by Sentry.
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[ppapapetrou76]

However, the current EventList return type will break anyway when upgrading to K8s 1.35 so, I'm considering the below approach:

This is not entirely true. There will be a breaking change when we upgrade the client to 1.35.
ArgoCD will work fine with K8s 1.35 server