@Override public int sizeInBytes(Span value) {
    V1Span v1Span = converter.convert(value);
    return v1SpanWriter.sizeInBytes(v1Span);
  }
This type isn't thread-safe: it re-uses state to avoid re-allocations in conversion loops.
zipkin/src/main/java/zipkin2/internal/V1JsonSpanWriter.java