code: public static <R> Supplier<R> spy(Supplier<R> supplier, Box<R> result) {
          return new CapturingSupplier<R>(supplier, result);
      }
docs: Proxies a supplier, spying for result.
  
  @param <R> the supplier result type
  @param supplier the supplier to be spied
  @param result a box that will be containing spied result
  @return the proxied supplier