conditionally register nfacct metrics and ensure nfacct counters

If the nfacct sub-system is not available in the kernel then:
1. nfacct based metrics won't be registered.
2. proxier will not attempt to ensure the counters

Signed-off-by: Daman Arora <aroradaman@gmail.com>

pkg/proxy/iptables/proxier.go

@@ -280,7 +280,7 @@ func NewProxier(ctx context.Context,
 	serviceHealthServer := healthcheck.NewServiceHealthServer(hostname, recorder, nodePortAddresses, healthzServer)
 	nfacctRunner, err := nfacct.New()
 	if err != nil {
-		logger.Error(err, "Failed to create nfacct runner")
+		logger.Error(err, "Failed to create nfacct runner, nfacct based metrics won't be available")
 	}
 
 	proxier := &Proxier{

pkg/proxy/metrics/metrics.go

@@ -299,8 +299,12 @@ func RegisterMetrics(mode kubeproxyconfig.ProxyMode) {
 
 		switch mode {
 		case kubeproxyconfig.ProxyModeIPTables:
-			legacyregistry.CustomMustRegister(iptablesCTStateInvalidDroppedMetricCollector)
-			legacyregistry.CustomMustRegister(localhostNodePortsAcceptedMetricsCollector)
+			if iptablesCTStateInvalidDroppedMetricCollector != nil {
+				legacyregistry.CustomMustRegister(iptablesCTStateInvalidDroppedMetricCollector)
+			}
+			if localhostNodePortsAcceptedMetricsCollector != nil {
+				legacyregistry.CustomMustRegister(localhostNodePortsAcceptedMetricsCollector)
+			}
 			legacyregistry.MustRegister(SyncFullProxyRulesLatency)
 			legacyregistry.MustRegister(SyncPartialProxyRulesLatency)
 			legacyregistry.MustRegister(IPTablesRestoreFailuresTotal)
@@ -332,6 +336,7 @@ func newNFAcctMetricCollector(counter string, description *metrics.Desc) *nfacct
 	client, err := nfacct.New()
 	if err != nil {
 		klog.ErrorS(err, "failed to initialize nfacct client")
+		return nil
 	}
 	return &nfacctMetricCollector{
 		client:      client,