[#426] engine: Support duration metrics

With `enable metrics` option, engine will collect
durations for all public methods.

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

pkg/local_object_storage/engine/exists.go

@@ -38,6 +38,10 @@ func (p *ExistsRes) Exists() bool {
 // Returns any error encountered that does not allow to
 // unambiguously determine the presence of an object.
 func (e *StorageEngine) Exists(prm *ExistsPrm) (*ExistsRes, error) {
+	if e.enableMetrics {
+		defer elapsed(existsDuration)()
+	}
+
 	exists, err := e.exists(prm.addr)
 
 	return &ExistsRes{