public String id() {
    return id;
  }
Unique 64bit identifier for this operation within the trace.

<p>Encoded as 16 lowercase hex characters. For example {@code ffdc9bb9a6453df3}

<p>A span is uniquely identified in storage by ({@linkplain #traceId}, {@linkplain #id()}).
zipkin/src/main/java/zipkin2/Span.java