diff --git a/metrics/label.go b/metrics/label.go
index 6c6b02ed7f..f59c149d02 100644
--- a/metrics/label.go
+++ b/metrics/label.go
@@ -1,11 +1,8 @@
 package metrics
 
-import "encoding/json"
-
 // Label hold an map[string]interface{} value that can be set arbitrarily.
 type Label interface {
 	Value() map[string]interface{}
-	String() string
 	Mark(map[string]interface{})
 }
 
@@ -26,8 +23,7 @@ func NewStandardLabel() *StandardLabel {
 
 // StandardLabel is the standard implementation of a Label.
 type StandardLabel struct {
-	value   map[string]interface{}
-	jsonStr string
+	value map[string]interface{}
 }
 
 // Value returns label values.
@@ -37,12 +33,5 @@ func (l *StandardLabel) Value() map[string]interface{} {
 
 // Mark records the label.
 func (l *StandardLabel) Mark(value map[string]interface{}) {
-	buf, _ := json.Marshal(value)
-	l.jsonStr = string(buf)
 	l.value = value
 }
-
-// String returns label by JSON format.
-func (l *StandardLabel) String() string {
-	return l.jsonStr
-}
diff --git a/metrics/prometheus/collector.go b/metrics/prometheus/collector.go
index 25c7b4f4c4..1921a0c37c 100644
--- a/metrics/prometheus/collector.go
+++ b/metrics/prometheus/collector.go
@@ -100,7 +100,11 @@ func (c *collector) addResettingTimer(name string, m metrics.ResettingTimer) {
 }
 
 func (c *collector) addLabel(name string, m metrics.Label) {
-	c.writeLabel(mutateKey(name), m.String())
+	labels := make([]string, 0, len(m.Value()))
+	for k, v := range m.Value() {
+		labels = append(labels, fmt.Sprintf(`%s="%s"`, mutateKey(k), fmt.Sprint(v)))
+	}
+	c.writeLabel(mutateKey(name), "{"+strings.Join(labels, ", ")+"}")
 }
 
 func (c *collector) writeLabel(name string, value interface{}) {
@@ -126,5 +130,7 @@ func (c *collector) writeSummaryPercentile(name, p string, value interface{}) {
 }
 
 func mutateKey(key string) string {
-	return strings.Replace(key, "/", "_", -1)
+	key = strings.Replace(key, "/", "_", -1)
+	key = strings.Replace(key, "-", "_", -1)
+	return key
 }