fail Gather'ing when label value is not utf8

prometheus/registry.go

@@ -23,6 +23,8 @@ import (
 
 	"github.com/golang/protobuf/proto"
 
+	"unicode/utf8"
+
 	dto "github.com/prometheus/client_model/go"
 )
 
@@ -679,6 +681,12 @@ func checkMetricConsistency(
 		)
 	}
 
+	for _, labelPair := range dtoMetric.GetLabel() {
+		if !utf8.ValidString(*labelPair.Value) {
+			return fmt.Errorf("collected metric's label %s is not utf8: %#v", *labelPair.Name, *labelPair.Value)
+		}
+	}
+
 	// Is the metric unique (i.e. no other metric with the same name and the same label values)?
 	h := hashNew()
 	h = hashAdd(h, metricFamily.GetName())

prometheus/registry_test.go

@@ -209,6 +209,34 @@ metric: <
 	expectedMetricFamilyMergedWithExternalAsProtoCompactText := []byte(`name:"name" help:"docstring" type:COUNTER metric:<label:<name:"constname" value:"constvalue" > label:<name:"labelname" value:"different_val" > counter:<value:42 > > metric:<label:<name:"constname" value:"constvalue" > label:<name:"labelname" value:"val1" > counter:<value:1 > > metric:<label:<name:"constname" value:"constvalue" > label:<name:"labelname" value:"val2" > counter:<value:1 > > 
 `)
 
+	externalMetricFamilyWithInvalidLabelValue := &dto.MetricFamily{
+		Name: proto.String("name"),
+		Help: proto.String("docstring"),
+		Type: dto.MetricType_COUNTER.Enum(),
+		Metric: []*dto.Metric{
+			{
+				Label: []*dto.LabelPair{
+					{
+						Name:  proto.String("constname"),
+						Value: proto.String("\xFF"),
+					},
+					{
+						Name:  proto.String("labelname"),
+						Value: proto.String("different_val"),
+					},
+				},
+				Counter: &dto.Counter{
+					Value: proto.Float64(42),
+				},
+			},
+		},
+	}
+
+	expectedMetricFamilyInvalidLabelValueAsText := []byte(`An error has occurred during metrics gathering:
+
+collected metric's label constname is not utf8: "\xff"
+`)
+
 	type output struct {
 		headers map[string]string
 		body    []byte
@@ -452,6 +480,22 @@ metric: <
 				externalMetricFamilyWithSameName,
 			},
 		},
+		{ // 16
+			headers: map[string]string{
+				"Accept": "application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=compact-text",
+			},
+			out: output{
+				headers: map[string]string{
+					"Content-Type": `text/plain; charset=utf-8`,
+				},
+				body: expectedMetricFamilyInvalidLabelValueAsText,
+			},
+			collector: metricVec,
+			externalMF: []*dto.MetricFamily{
+				externalMetricFamily,
+				externalMetricFamilyWithInvalidLabelValue,
+			},
+		},
 	}
 	for i, scenario := range scenarios {
 		registry := prometheus.NewPedanticRegistry()