[#451] metrics: Move to internal

`metrics` don't look like something others want to import.

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

internal/metrics/morphcache.go

@@ -0,0 +1,39 @@
+package metrics
+
+import (
+	"strconv"
+	"time"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-observability/metrics"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type MorphCacheMetrics interface {
+	AddMethodDuration(method string, success bool, d time.Duration)
+}
+
+type morphCacheMetrics struct {
+	methodDuration *prometheus.HistogramVec
+}
+
+var _ MorphCacheMetrics = (*morphCacheMetrics)(nil)
+
+func newMorphCacheMetrics(ns string) *morphCacheMetrics {
+	return &morphCacheMetrics{
+		methodDuration: metrics.NewHistogramVec(prometheus.HistogramOpts{
+			Namespace: ns,
+			Subsystem: morphCacheSubsystem,
+			Name:      "request_duration_seconds",
+			Help:      "Morph cache request process duration",
+		}, []string{successLabel, methodLabel}),
+	}
+}
+
+func (m *morphCacheMetrics) AddMethodDuration(method string, success bool, d time.Duration) {
+	m.methodDuration.With(
+		prometheus.Labels{
+			successLabel: strconv.FormatBool(success),
+			methodLabel:  method,
+		},
+	).Observe(d.Seconds())
+}