[#1695] qos: Add treesync tag

Tree sync is too much different from GC and rebuild to use the same tag for GC and tree sync.

Change-Id: Ib44d5fa9a88daff507d759d0b0410cc9272e236f
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/qos/stats.go

@@ -9,6 +9,7 @@ var statTags = map[string]struct{}{
 	IOTagPolicer.String():    {},
 	IOTagWritecache.String(): {},
 	IOTagCritical.String():   {},
+	IOTagTreeSync.String():   {},
 	unknownStatsTag:          {},
 }