void writeSpan(Span span, int sizeOfSpan, WriteBuffer result) {
    result.writeByte(SPAN.key);
    result.writeVarint(sizeOfSpan); // length prefix
    SPAN.writeValue(result, span);
  }
Encodes per ListOfSpans data wireType, where field one is repeated spans
zipkin/src/main/java/zipkin2/internal/Proto3SpanWriter.java