Review feedback: add comment and tests for WrapRegistererWith.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

prometheus/promauto/auto_test.go

@@ -19,9 +19,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-func TestWrapNil(t *testing.T) {
-	// A nil registerer should be treated as a no-op by promauto, even when wrapped.
-	registerer := prometheus.WrapRegistererWith(prometheus.Labels{"foo": "bar"}, nil)
-	c := With(registerer).NewCounter(prometheus.CounterOpts{Name: "test"})
-	c.Inc()
+func TestNil(t *testing.T) {
+	// A nil registerer should be treated as a no-op by promauto.
+	With(nil).NewCounter(prometheus.CounterOpts{Name: "test"}).Inc()
 }

prometheus/wrap.go

@@ -28,7 +28,8 @@ import (
 // registered with the wrapped Registerer in a modified way. The modified
 // Collector adds the provided Labels to all Metrics it collects (as
 // ConstLabels). The Metrics collected by the unmodified Collector must not
-// duplicate any of those labels.
+// duplicate any of those labels. Wrapping a nil value is valid, resulting
+// in a no-op Registerer.
 //
 // WrapRegistererWith provides a way to add fixed labels to a subset of
 // Collectors. It should not be used to add fixed labels to all metrics exposed.
@@ -51,6 +52,7 @@ func WrapRegistererWith(labels Labels, reg Registerer) Registerer {
 // Registerer. Collectors registered with the returned Registerer will be
 // registered with the wrapped Registerer in a modified way. The modified
 // Collector adds the provided prefix to the name of all Metrics it collects.
+// Wrapping a nil value is valid, resulting in a no-op Registerer.
 //
 // WrapRegistererWithPrefix is useful to have one place to prefix all metrics of
 // a sub-system. To make this work, register metrics of the sub-system with the

prometheus/wrap_test.go

@@ -321,3 +321,12 @@ func TestWrap(t *testing.T) {
 	}
 
 }
+
+func TestNil(t *testing.T) {
+	// A wrapped nil registerer should be treated as a no-op, and not panic.
+	c := NewCounter(CounterOpts{Name: "test"})
+	err := WrapRegistererWith(Labels{"foo": "bar"}, nil).Register(c)
+	if err != nil {
+		t.Fatal("registering failed:", err)
+	}
+}