code: func (SpanSerializer) Serialize(sms []*zipkinmodel.SpanModel) (protoBlob []byte, err error) {
  	var listOfSpans ListOfSpans
  
  	for _, sm := range sms {
  		sp, err := modelSpanToProtoSpan(sm)
  		if err != nil {
  			return nil, err
  		}
  		listOfSpans.Spans = append(listOfSpans.Spans, sp)
  	}
  
  	return proto.Marshal(&listOfSpans)
  }
docs: // Serialize takes an array of zipkin SpanModel objects and serializes it to a protobuf blob.