code: @Override
    public Runnable wrap(Runnable runnable) {
      if (isTracing()) {
        return new SpanContinuingTraceRunnable(this, this.traceKeys, this.spanNamer, runnable);
      }
      return runnable;
    }
docs: Wrap the runnable in a TraceRunnable, if tracing.
  
  @return The runnable provided, wrapped if tracing, 'runnable' if not.