public String traceId() {
    return traceId;
  }
Trace identifier, set on all spans within it.

<p>Encoded as 16 or 32 lowercase hex characters corresponding to 64 or 128 bits. For example,
a 128bit trace ID looks like {@code 4e441824ec2b6a44ffdc9bb9a6453df3}.

<p>Some systems downgrade trace identifiers to 64bit by dropping the left-most 16 characters.
For example, {@code 4e441824ec2b6a44ffdc9bb9a6453df3} becomes {@code ffdc9bb9a6453df3}.
zipkin/src/main/java/zipkin2/Span.java