Add const labels to counter.

prometheus/counter.go

@@ -55,7 +55,7 @@ func NewCounter(opts CounterOpts) Counter {
 		nil,
 		opts.ConstLabels,
 	)
-	result := &counter{value: value{desc: desc, valType: CounterValue}}
+	result := &counter{value: value{desc: desc, valType: CounterValue, labelPairs: desc.constLabelPairs}}
 	result.Init(result) // Init self-collection.
 	return result
 }

prometheus/counter_test.go

@@ -13,12 +13,17 @@
 
 package prometheus
 
-import "testing"
+import (
+	"testing"
+
+	dto "github.com/prometheus/client_model/go"
+)
 
 func TestCounterAdd(t *testing.T) {
 	counter := NewCounter(CounterOpts{
 		Name:        "test",
 		Help:        "test help",
+		ConstLabels: Labels{"a": "1", "b": "2"},
 	}).(*counter)
 	counter.Inc()
 	if expected, got := 1., counter.val; expected != got {
@@ -32,6 +37,13 @@ func TestCounterAdd(t *testing.T) {
 	if expected, got := "counter cannot decrease in value", decreaseCounter(counter).Error(); expected != got {
 		t.Errorf("Expected error %q, got %q.", expected, got)
 	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	if expected, got := `label:<name:"a" value:"1" > label:<name:"b" value:"2" > counter:<value:43 > `, m.String(); expected != got {
+		t.Errorf("expected %q, got %q", expected, got)
+	}
 }
 
 func decreaseCounter(c *counter) (err error) {