code: @SuppressWarnings("unchecked")
      public static <T extends Number> FlowableTransformer<T, Statistics> collectStats() {
          return (FlowableTransformer<T, Statistics>) CollectStatsHolder.INSTANCE;
      }
docs: <p>
  Converts a stream of {@code Number} to a stream of {@link Statistics} about
  those numbers.
  
  <p>
  <img src=
  "https://raw.githubusercontent.com/davidmoten/rxjava2-extras/master/src/docs/collectStats.png"
  alt="image">
  
  @param <T>
  item type
  @return transformer that converts a stream of Number to a stream of
  Statistics