Add structured reporting diff to WorkstationCluster (#6700)

### BRIEF Change description

Fixes #6630

#### WHY do we need this change?

Add structured reporting diff to the controller in
`pkg/controller/direct/workstations/cluster_controller.go`.
The `structuredreporting.ReportDiff` should be used in the `Update`
method of the adapter to report which fields are being updated.
This helps in debugging reconciliation loops and provides better
visibility into what changed.

#### Special notes for your reviewer:

#### Does this PR add something which needs to be 'release noted'?
```release-note
NONE
```

#### Additional documentation e.g., references, usage docs, etc.:
```docs
NONE
```

#### Intended Milestone
- [ ] Reviewer tagged PR with the actual milestone.

### Tests you have done

- [ ] Run `make ready-pr` to ensure this PR is ready for review.
- [ ] Perform necessary E2E testing for changed resources.

Author: anhdle-sso

pkg/controller/direct/workstations/cluster_controller.go

@@ -26,6 +26,7 @@ import (
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/controller/direct/directbase"
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/controller/direct/registry"
 	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/fuzztesting"
+	"github.com/GoogleCloudPlatform/k8s-config-connector/pkg/structuredreporting"
 
 	gcp "cloud.google.com/go/workstations/apiv1"
 	pb "cloud.google.com/go/workstations/apiv1/workstationspb"
@@ -251,18 +252,24 @@ func (a *Adapter) Update(ctx context.Context, updateOp *directbase.UpdateOperati
 	}
 	resource.Name = a.id.FullyQualifiedName()
 
+	report := &structuredreporting.Diff{Object: updateOp.GetUnstructured()}
+
 	updateMask := &fieldmaskpb.FieldMask{}
 	if !reflect.DeepEqual(resource.Annotations, a.actual.Annotations) {
+		report.AddField("annotations", a.actual.Annotations, resource.Annotations)
 		updateMask.Paths = append(updateMask.Paths, "annotations")
 	}
 	if !reflect.DeepEqual(resource.Labels, a.actual.Labels) {
+		report.AddField("labels", a.actual.Labels, resource.Labels)
 		updateMask.Paths = append(updateMask.Paths, "labels")
 	}
 
 	if len(updateMask.Paths) == 0 {
 		return nil
 	}
 
+	structuredreporting.ReportDiff(ctx, report)
+
 	req := &pb.UpdateWorkstationClusterRequest{
 		UpdateMask:         updateMask,
 		WorkstationCluster: resource,