[#29] metrics: Use map for constant labels

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

metrics/desc.go

@@ -74,22 +74,17 @@ type Description struct {
 	Subsystem      string
 	Name           string
 	Help           string
-	ConstantLabels []KeyValue
+	ConstantLabels prometheus.Labels
 	VariableLabels []string
 }
 
-type KeyValue struct {
-	Key   string `json:"key"`
-	Value string `json:"value"`
-}
-
 func (d *Description) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&struct {
 		Type           string            `json:"type"`
 		FQName         string            `json:"name"`
 		Help           string            `json:"help"`
-		ConstantLabels []KeyValue `json:"constant_labels"`
-		VariableLabels []string   `json:"variable_labels"`
+		ConstantLabels prometheus.Labels `json:"constant_labels,omitempty"`
+		VariableLabels []string          `json:"variable_labels,omitempty"`
 	}{
 		Type:           d.Type.String(),
 		FQName:         d.BuildFQName(),
@@ -103,14 +98,6 @@ func (d *Description) BuildFQName() string {
 	return prometheus.BuildFQName(d.Namespace, d.Subsystem, d.Name)
 }
 
-func (d *Description) ConstLabelsMap() map[string]string {
-	constsLabels := make(map[string]string, len(d.ConstantLabels))
-	for _, kv := range d.ConstantLabels {
-		constsLabels[kv.Key] = kv.Value
-	}
-	return constsLabels
-}
-
 // DescribeAll returns descriptions for metrics.
 func DescribeAll() []Description {
 	var list []Description
@@ -129,7 +116,7 @@ func newOpts(description Description) prometheus.Opts {
 		Subsystem:   description.Subsystem,
 		Name:        description.Name,
 		Help:        description.Help,
-		ConstLabels: description.ConstLabelsMap(),
+		ConstLabels: description.ConstantLabels,
 	}
 }