code: func (s *Segment) addMetadata(key string, value interface{}) {
  	s.Lock()
  	defer s.Unlock()
  
  	if s.Metadata == nil {
  		s.Metadata = make(map[string]map[string]interface{})
  		s.Metadata["default"] = make(map[string]interface{})
  	}
  	s.Metadata["default"][key] = value
  }
docs: // addMetadata adds a key-value pair that can be queried by AWS X-Ray.
  // AWS X-Ray only supports annotations of type string, integer or boolean.