mocktracer/mockspan.go
func (s *MockSpan) BaggageItem(key string) string {
	s.RLock()
	defer s.RUnlock()
	return s.SpanContext.Baggage[key]
}
func (s *MockSpan) BaggageItem(key string) string {
	s.RLock()
	defer s.RUnlock()
	return s.SpanContext.Baggage[key]
}
// BaggageItem belongs to the Span interface