public static Builder newBuilder() {
    return new Builder();
  }
True if we are contributing to a span started by another tracer (ex on a different host).
Defaults to null. When set, it is expected for {@link #kind()} to be {@link Kind#SERVER}.

<p>When an RPC trace is client-originated, it will be sampled and the same span ID is used for
the server side. However, the server shouldn't set span.timestamp or duration since it didn't
start the span.
zipkin/src/main/java/zipkin2/Span.java