mocktracer/mockspan.go
func (s *MockSpan) Tag(k string) interface{} {
	s.RLock()
	defer s.RUnlock()
	return s.tags[k]
}
func (s *MockSpan) Tag(k string) interface{} {
	s.RLock()
	defer s.RUnlock()
	return s.tags[k]
}
// Tag returns a single tag