[#426] Put prometheus behind pkg/metrics

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/metrics/metrics.go

@@ -0,0 +1,21 @@
+package metrics
+
+const namespace = "neofs_node"
+
+type StorageMetrics struct {
+	objectServiceMetrics
+	engineMetrics
+}
+
+func NewStorageMetrics() *StorageMetrics {
+	objectService := newObjectServiceMetrics()
+	objectService.register()
+
+	engine := newEngineMetrics()
+	engine.register()
+
+	return &StorageMetrics{
+		objectServiceMetrics: objectService,
+		engineMetrics:        engine,
+	}
+}