code: public static <M extends Multimap<?, ?>> MultimapJsonSerializer<M, ?, ?> newInstance( KeySerializer<?> keySerializer,
                                                                                            JsonSerializer<?> valueSerializer ) {
          return new MultimapJsonSerializer( keySerializer, valueSerializer );
      }
docs: @param keySerializer {@link KeySerializer} used to serialize the keys.
  @param valueSerializer {@link JsonSerializer} used to serialize the values.
  @param <M> Type of the {@link Multimap}
  
  @return a new instance of {@link MultimapJsonSerializer}