Improve resliency of size tagging when hostedcluster KAS down

When the kube apiserver of a hosted cluster is not available, the
replica status of nodepools will not be accurate because the CAPI
controllers can no longer get node counts from the API server. This
commit improves the handling of this situation with 2 changes:
- Switches to use .spec.replicas to determine node count of nodepools
  that do not have autoscaling turned on.
- Once a hosted cluster has been tagged with a size, only if the kube
  apiserver of the hosted cluster is available is the hosted cluster
  allowed to move to a different size.

hypershift-operator/controllers/hostedclustersizing/hostedclustersizing_controller.go

@@ -15,6 +15,7 @@ import (
 	hyperutil "github.com/openshift/hypershift/support/util"
 	corev1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/errors"
+	"k8s.io/apimachinery/pkg/api/meta"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/types"
 	"k8s.io/apimachinery/pkg/util/sets"
@@ -218,6 +219,12 @@ func (r *reconciler) reconcile(
 		return nil, fmt.Errorf("failed to determine if HCCO reports node count: %w", err)
 	}
 
+	// Determine if the Kube API Server is available to determine if we can trust the node count from the HCCO or nodepool.status.replicas
+	// If the Kube API Server is not available, we cannot trust the node count from the HCCO or nodepool.status.replicas
+	// Ref: kubernetes-sigs/cluster-api#10195
+	kasAvailableCondition := meta.FindStatusCondition(hostedCluster.Status.Conditions, string(hypershiftv1beta1.KubeAPIServerAvailable))
+	kasAvailable := kasAvailableCondition != nil && kasAvailableCondition.Status == metav1.ConditionTrue
+
 	// first, we figure out the node count for the hosted cluster
 	var nodeCount uint32
 	if hccoReportsNodeCount {
@@ -236,7 +243,18 @@ func (r *reconciler) reconcile(
 		}
 
 		for _, nodePool := range nodePools.Items {
-			nodeCount += uint32(nodePool.Status.Replicas)
+			var replicas uint32
+			// If autoscaling, the replicas should be returned from status
+			if nodePool.Spec.AutoScaling != nil {
+				// If the Kube API Server is not available, and we already have a size label, skip processing
+				if !kasAvailable && sizeClassLabelPresent {
+					return nil, nil
+				}
+				replicas = uint32(nodePool.Status.Replicas)
+			} else if nodePool.Spec.Replicas != nil {
+				replicas = uint32(*nodePool.Spec.Replicas)
+			}
+			nodeCount += replicas
 		}
 	}