public List<Span> convert(V1Span source) {
    List<Span> out = new ArrayList<>();
    convert(source, out);
    return out;
  }
Allows you to split a v1 span when necessary. This can be the case when reading merged
client+server spans from storage or parsing old data formats.

<p>This type isn't thread-safe: it re-uses state to avoid re-allocations in conversion loops.
zipkin/src/main/java/zipkin2/v1/V1SpanConverter.java