k8s.io/component-base: support GetCounterMetricValue for Counter

GetCounterMetricValue has an unchecked conversion to metrics.Metric,
something that didn't work for a *Counter because it didn't implement
that interface.

staging/src/k8s.io/component-base/metrics/counter.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"github.com/blang/semver"
 	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
 )
 
 // Counter is our internal representation for our wrapping struct around prometheus
@@ -31,6 +32,9 @@ type Counter struct {
 	selfCollector
 }
 
+// The implementation of the Metric interface is expected by testutil.GetCounterMetricValue.
+var _ Metric = &Counter{}
+
 // NewCounter returns an object which satisfies the kubeCollector and CounterMetric interfaces.
 // However, the object returned will not measure anything unless the collector is first
 // registered, since the metric is lazily instantiated.
@@ -46,6 +50,14 @@ func NewCounter(opts *CounterOpts) *Counter {
 	return kc
 }
 
+func (c *Counter) Desc() *prometheus.Desc {
+	return c.metric.Desc()
+}
+
+func (c *Counter) Write(to *dto.Metric) error {
+	return c.metric.Write(to)
+}
+
 // Reset resets the underlying prometheus Counter to start counting from 0 again
 func (c *Counter) Reset() {
 	if !c.IsCreated() {