trace: document that SetLabel should be called before Finish/FinishWait
@@ -666,6 +666,8 @@ func (s *Span) TraceID() string {
// If a label is given a value automatically and by SetLabel, the
// automatically-set value is used.
// If s is nil, does nothing.
+//
+// SetLabel shouldn't be called after Finish or FinishWait.
func (s *Span) SetLabel(key, value string) {
if s == nil {
return
