[#373] metrics: Move labels to consts

To unify label naming all lable keys and other consts are moved to
one file.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/metrics/morphcache.go

@@ -8,12 +8,6 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-const (
-	mcSubsystem = "morphcache"
-	mcSuccess   = "success"
-	mcMethod    = "method"
-)
-
 type MorphCacheMetrics interface {
 	AddMethodDuration(method string, success bool, d time.Duration)
 }
@@ -32,18 +26,18 @@ func newMorphCacheMetrics(ns string) *morphCacheMetrics {
 	return &morphCacheMetrics{
 		methodDuration: metrics.NewHistogramVec(prometheus.HistogramOpts{
 			Namespace: ns,
-			Subsystem: mcSubsystem,
+			Subsystem: morphCacheSubsystem,
 			Name:      "request_duration_seconds",
 			Help:      "Morph cache request process duration",
-		}, []string{mcSuccess, mcMethod}),
+		}, []string{successLabel, methodLabel}),
 	}
 }
 
 func (m *morphCacheMetrics) AddMethodDuration(method string, success bool, d time.Duration) {
 	m.methodDuration.With(
 		prometheus.Labels{
-			mcSuccess: strconv.FormatBool(success),
-			mcMethod:  method,
+			successLabel: strconv.FormatBool(success),
+			methodLabel:  method,
 		},
 	).Observe(d.Seconds())
 }