Span.Builder newSpanBuilder(V1Span source, Endpoint e) {
    Span.Builder result = newBuilder(Span.newBuilder(), source).localEndpoint(e);
    spans.add(result);
    return result;
  }
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