code: public static Bson mul(final String fieldName, final Number number) {
          notNull("number", number);
          return new SimpleUpdate<Number>(fieldName, number, "$mul");
      }
docs: Creates an update that multiplies the value of the field with the given name by the given number.
  
  @param fieldName the non-null field name
  @param number    the non-null number
  @return the update
  @mongodb.driver.manual reference/operator/update/mul/ $mul