Change prometheus metrics type from summary to histogram

Resolves issue: #905
The summary types are tagged for deprecation,
Kubernetes recommended to use histograms instead of summaries.
The main advantages of histogram types are aggregation and inexpensive.
In this commit, we changed three Antrea controller metrics from summary
to histogram type. They are DurationAppliedToGroupSyncing,
DurationAddressGroupSyncing, and DurationInternalNetworkPolicySyncing.

pkg/controller/metrics/prometheus.go

@@ -38,20 +38,20 @@ var (
 		Help:           "The total number of internal-networkpolicy processed",
 		StabilityLevel: metrics.STABLE,
 	})
-	DurationAppliedToGroupSyncing = metrics.NewSummary(&metrics.SummaryOpts{
+	DurationAppliedToGroupSyncing = metrics.NewHistogram(&metrics.HistogramOpts{
 		Name:           "antrea_controller_applied_to_group_sync_duration_milliseconds",
 		Help:           "The duration of syncing applied-to-group",
-		StabilityLevel: metrics.STABLE,
+		StabilityLevel: metrics.ALPHA,
 	})
-	DurationAddressGroupSyncing = metrics.NewSummary(&metrics.SummaryOpts{
+	DurationAddressGroupSyncing = metrics.NewHistogram(&metrics.HistogramOpts{
 		Name:           "antrea_controller_address_group_sync_duration_milliseconds",
 		Help:           "The duration of syncing address-group",
-		StabilityLevel: metrics.STABLE,
+		StabilityLevel: metrics.ALPHA,
 	})
-	DurationInternalNetworkPolicySyncing = metrics.NewSummary(&metrics.SummaryOpts{
+	DurationInternalNetworkPolicySyncing = metrics.NewHistogram(&metrics.HistogramOpts{
 		Name:           "antrea_controller_network_policy_sync_duration_milliseconds",
 		Help:           "The duration of syncing internal-networkpolicy",
-		StabilityLevel: metrics.STABLE,
+		StabilityLevel: metrics.ALPHA,
 	})
 	LengthAppliedToGroupQueue = metrics.NewGauge(&metrics.GaugeOpts{
 		Name:           "antrea_controller_length_applied_to_group_queue",