code: public static <T> RateLimiterOperator<T> of(RateLimiter rateLimiter) {
          return of(rateLimiter, Schedulers.parallel());
      }
docs: Creates a RateLimiterOperator.
  
  @param <T>         the value type of the upstream and downstream
  @param rateLimiter the Rate limiter
  @return a RateLimiterOperator