fix HPA plain metric calculate

fix inconsistency in plain metric replica calculator when usageRatio == 1.0, keep same with resource replicas

pkg/controller/podautoscaler/replica_calculator.go

@@ -214,7 +214,7 @@ func (c *ReplicaCalculator) calcPlainMetricReplicas(metrics metricsclient.PodMet
 			for podName := range missingPods {
 				metrics[podName] = metricsclient.PodMetric{Value: targetUsage}
 			}
-		} else {
+		} else if usageRatio > 1.0 {
 			// on a scale-up, treat missing pods as using 0% of the resource request
 			for podName := range missingPods {
 				metrics[podName] = metricsclient.PodMetric{Value: 0}

pkg/controller/podautoscaler/replica_calculator_test.go

@@ -1322,6 +1322,21 @@ func TestReplicaCalcEmptyCPURequest(t *testing.T) {
 	tc.runTest(t)
 }
 
+func TestPlainMetricReplicaCalcMissingMetricsNoChangeEq(t *testing.T) {
+	tc := replicaCalcTestCase{
+		currentReplicas:  5,
+		expectedReplicas: 5,
+		metric: &metricInfo{
+			name:          "qps",
+			levels:        []int64{20000, 19000, 21000},
+			targetUsage:   20000,
+			expectedUsage: 20000,
+			metricType:    podMetric,
+		},
+	}
+	tc.runTest(t)
+}
+
 func TestReplicaCalcMissingMetricsNoChangeEq(t *testing.T) {
 	tc := replicaCalcTestCase{
 		currentReplicas:  2,