code: public final <K> Mono<Map<K, Collection<T>>> collectMultimap(Function<? super T, ? extends K> keyExtractor) {
  		return collectMultimap(keyExtractor, identityFunction());
  	}
docs: Collect all elements emitted by this {@link Flux} into a {@link Map multimap} that is
  emitted by the resulting {@link Mono} when this sequence completes.
  The key is extracted from each element by applying the {@code keyExtractor}
  {@link Function}, and every element mapping to the same key is stored in the {@link List}
  associated to said key.
  
  <p>
  <img class="marble" src="doc-files/marbles/collectMultiMapWithKeyExtractor.svg" alt="">
  
  @reactor.discard This operator discards the buffer upon cancellation or error triggered by a data signal.
  
  @param keyExtractor a {@link Function} to map elements to a key for the {@link Map}
  
  @param <K> the type of the key extracted from each source element
  @return a {@link Mono} of a {@link Map} of key-List(elements) pairs