[#1367] fstree: Add size to file counter

FSTree file counter used by writecache. As writecache has now only one
storage, so it is required to use real object size to get writecache
size more accurate than `count * max_object_size`.

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

pkg/local_object_storage/writecache/cache.go

@@ -29,6 +29,8 @@ type cache struct {
 	wg sync.WaitGroup
 	// fsTree contains big files stored directly on file-system.
 	fsTree *fstree.FSTree
+	// counter contains atomic counters for the number of objects stored in cache.
+	counter *fstree.SimpleCounter
 }
 
 // wcStorageType is used for write-cache operations logging.
@@ -56,6 +58,7 @@ func New(opts ...Option) Cache {
 	c := &cache{
 		flushCh: make(chan objectInfo),
 		mode:    mode.Disabled,
+		counter: fstree.NewSimpleCounter(),
 
 		options: options{
 			log:             &logger.Logger{Logger: zap.NewNop()},