Expose failure to detect duplicate label names

This is #471.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/registry_test.go

@@ -308,6 +308,29 @@ collected metric named "complex_count" collides with previously collected summar
 
 collected metric named "complex_count" collides with previously collected histogram named "complex"
 `)
+	externalMetricFamilyWithDuplicateLabel := &dto.MetricFamily{
+		Name: proto.String("broken_metric"),
+		Help: proto.String("The registry should detect the duplicate label."),
+		Type: dto.MetricType_COUNTER.Enum(),
+		Metric: []*dto.Metric{
+			{
+				Label: []*dto.LabelPair{
+					{
+						Name:  proto.String("foo"),
+						Value: proto.String("bar"),
+					},
+					{
+						Name:  proto.String("foo"),
+						Value: proto.String("baz"),
+					},
+				},
+				Counter: &dto.Counter{
+					Value: proto.Float64(2.7),
+				},
+			},
+		},
+	}
+	duplicateLabelMsg := []byte(`duplicate label mumble mumble`)
 
 	type output struct {
 		headers map[string]string
@@ -646,6 +669,20 @@ collected metric named "complex_count" collides with previously collected histog
 				externalMetricFamilyWithBucketSuffix,
 			},
 		},
+		{ // 22
+			headers: map[string]string{
+				"Accept": "text/plain",
+			},
+			out: output{
+				headers: map[string]string{
+					"Content-Type": `text/plain; version=0.0.4; charset=utf-8`,
+				},
+				body: duplicateLabelMsg,
+			},
+			externalMF: []*dto.MetricFamily{
+				externalMetricFamilyWithDuplicateLabel,
+			},
+		},
 	}
 	for i, scenario := range scenarios {
 		registry := prometheus.NewPedanticRegistry()