mocktracer/mockspan.go
func (s *MockSpan) SetBaggageItem(key, val string) opentracing.Span {
	s.Lock()
	defer s.Unlock()
	s.SpanContext = s.SpanContext.WithBaggageItem(key, val)
	return s
}
func (s *MockSpan) SetBaggageItem(key, val string) opentracing.Span {
	s.Lock()
	defer s.Unlock()
	s.SpanContext = s.SpanContext.WithBaggageItem(key, val)
	return s
}
// SetBaggageItem belongs to the Span interface