code: public static String toUnique(Span span) {
  
          String id = span.getId();
          if (span.clientSpan()) {
              id = getClientId(span.getId());
          }
  
          return id;
      }
docs: Utility method to get unique id of the span. Note that method
  does not change the span id.
  
  @param span Span.
  @return Unique id of the span.