public byte[] write(Span onlySpan) {
    byte[] result = new byte[sizeInBytes(onlySpan)];
    write(onlySpan, WriteBuffer.wrap(result));
    return result;
  }
This type isn't thread-safe: it re-uses state to avoid re-allocations in conversion loops.
zipkin/src/main/java/zipkin2/internal/V1ThriftSpanWriter.java