fix(descheduler): reset prometheus usage client at each extension point

Profile creation was moved outside the descheduling cycle in b214c147,
but reconcileInClusterSAToken() still runs only in runFnc(), after
newDescheduler() returns. This leaves the prometheus client nil when
LowNodeUtilization's New() runs, causing "prometheus client not
initialized" at startup.

Avoid failing at plugin creation time if the prometheus
client is not yet available. Instead, usageClientForMetrics() is now
called at the start of every extension point via a resetUsageClient()
helper, so each descheduling cycle picks up the latest client regardless
of when the SA token is reconciled or rotated.

Fixes: https://github.com/kubernetes-sigs/descheduler/issues/1840

Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>

pkg/framework/plugins/nodeutilization/lownodeutilization_test.go

@@ -1661,3 +1661,83 @@ func TestLowNodeUtilizationWithPrometheusMetrics(t *testing.T) {
 		t.Run(tc.name, testFnc(false, tc.expectedPodsEvicted))
 	}
 }
+
+// TestLowNodeUtilizationPrometheusClientNilAtCreation ensures the plugin
+// tolerates a nil prometheus client at creation time (e.g. in-cluster SA token
+// not yet reconciled) and correctly picks up the client exposed by the handle
+// at Balance call time.
+func TestLowNodeUtilizationPrometheusClientNilAtCreation(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	n1NodeName := "n1"
+	n2NodeName := "n2"
+	n3NodeName := "n3"
+
+	nodes := []*v1.Node{
+		test.BuildTestNode(n1NodeName, 4000, 3000, 9, nil),
+		test.BuildTestNode(n2NodeName, 4000, 3000, 10, nil),
+		test.BuildTestNode(n3NodeName, 4000, 3000, 10, nil),
+	}
+	pods := []*v1.Pod{
+		test.BuildTestPod("p1", 400, 0, n1NodeName, test.SetRSOwnerRef),
+		test.BuildTestPod("p2", 400, 0, n1NodeName, test.SetRSOwnerRef),
+		test.BuildTestPod("p3", 400, 0, n1NodeName, test.SetRSOwnerRef),
+		test.BuildTestPod("p4", 400, 0, n1NodeName, test.SetRSOwnerRef),
+		test.BuildTestPod("p5", 400, 0, n1NodeName, test.SetRSOwnerRef),
+		test.BuildTestPod("p6", 400, 0, n2NodeName, test.SetRSOwnerRef),
+	}
+
+	var objs []runtime.Object
+	for _, node := range nodes {
+		objs = append(objs, node)
+	}
+	for _, pod := range pods {
+		objs = append(objs, pod)
+	}
+	fakeClient := fake.NewSimpleClientset(objs...)
+
+	handle, podEvictor, err := frameworktesting.InitFrameworkHandle(
+		ctx, fakeClient, nil, defaultevictor.DefaultEvictorArgs{NodeFit: true}, nil,
+	)
+	if err != nil {
+		t.Fatalf("Unable to initialize a framework handle: %v", err)
+	}
+
+	// PrometheusClientImpl is intentionally nil here — the in-cluster SA token
+	// has not been reconciled yet at plugin-creation time.
+	args := &LowNodeUtilizationArgs{
+		Thresholds:       api.ResourceThresholds{MetricResource: 30},
+		TargetThresholds: api.ResourceThresholds{MetricResource: 50},
+		MetricsUtilization: &MetricsUtilization{
+			Source: api.PrometheusMetrics,
+			Prometheus: &Prometheus{
+				Query: "instance:node_cpu:rate:sum",
+			},
+		},
+	}
+
+	plugin, err := NewLowNodeUtilization(ctx, args, handle)
+	if err != nil {
+		t.Fatalf("plugin creation must succeed even when prometheus client is nil: %v", err)
+	}
+
+	// Simulate the SA token becoming available before the first descheduling cycle.
+	handle.PrometheusClientImpl = &fakePromClient{
+		result: model.Vector{
+			sample("instance:node_cpu:rate:sum", n1NodeName, 0.57), // over target
+			sample("instance:node_cpu:rate:sum", n2NodeName, 0.42), // over target
+			sample("instance:node_cpu:rate:sum", n3NodeName, 0.20), // under threshold
+		},
+		dataType: model.ValVector,
+	}
+
+	status := plugin.(frameworktypes.BalancePlugin).Balance(ctx, nodes)
+	if status != nil {
+		t.Fatalf("Balance failed: %v", status.Err)
+	}
+
+	if podEvictor.TotalEvicted() == 0 {
+		t.Error("expected at least one pod to be evicted")
+	}
+}