[#563] writecache: Fix metrics and bolt delete

Estimate cache size after delete objects to update metric.
Update counters on small object deletion.
Do not count bbolt DB file as FSTree object.

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

pkg/local_object_storage/writecache/writecachebbolt/state.go

@@ -68,6 +68,9 @@ func (c *cache) initCounters() error {
 	if err != nil {
 		return fmt.Errorf("could not read write-cache FS counter: %w", err)
 	}
+	if inFS > 0 {
+		inFS-- //small.bolt DB file
+	}
 
 	c.objCounters.cDB.Store(inDB)
 	c.objCounters.cFS.Store(inFS)