code: public EntryStream<K, V> filterKeys(Predicate<? super K> keyPredicate) {
          return filter(e -> keyPredicate.test(e.getKey()));
      }
docs: Returns a stream consisting of the elements of this stream which keys
  match the given predicate.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param keyPredicate a non-interfering, stateless predicate to apply to
  the key of each element to determine if it should be included
  @return the new stream