discovery(k8s): Only register client-go metrics adapters when needed

Previously the metrics adapters for client-go were registered in an init function.
This resulted in clobbering default metrics providers when these packages are imported
into an application that leverages the default client-go metrics registry.

Instead, let's only register these adapters when requested.

Signed-off-by: Stephen Heckler <sheckler@cloudflare.com>

discovery/metrics.go

@@ -19,16 +19,6 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-var (
-	clientGoRequestMetrics  = &clientGoRequestMetricAdapter{}
-	clientGoWorkloadMetrics = &clientGoWorkqueueMetricsProvider{}
-)
-
-func init() {
-	clientGoRequestMetrics.RegisterWithK8sGoClient()
-	clientGoWorkloadMetrics.RegisterWithK8sGoClient()
-}
-
 // Metrics to be used with a discovery manager.
 type Metrics struct {
 	FailedConfigs     prometheus.Gauge

discovery/metrics_k8s_client.go

@@ -35,6 +35,11 @@ const (
 	workqueueMetricsNamespace  = KubernetesMetricsNamespace + "_workqueue"
 )
 
+var (
+	clientGoRequestMetrics  = &clientGoRequestMetricAdapter{}
+	clientGoWorkloadMetrics = &clientGoWorkqueueMetricsProvider{}
+)
+
 var (
 	// Metrics for client-go's HTTP requests.
 	clientGoRequestResultMetricVec = prometheus.NewCounterVec(
@@ -135,6 +140,9 @@ func clientGoMetrics() []prometheus.Collector {
 }
 
 func RegisterK8sClientMetricsWithPrometheus(registerer prometheus.Registerer) error {
+	clientGoRequestMetrics.RegisterWithK8sGoClient()
+	clientGoWorkloadMetrics.RegisterWithK8sGoClient()
+
 	for _, collector := range clientGoMetrics() {
 		err := registerer.Register(collector)
 		if err != nil {