code: public static Bson exists(final String fieldName, final boolean exists) {
          return new OperatorFilter<BsonBoolean>("$exists", fieldName, BsonBoolean.valueOf(exists));
      }
docs: Creates a filter that matches all documents that either contain or do not contain the given field, depending on the value of the
  exists parameter.
  
  @param fieldName the field name
  @param exists    true to check for existence, false to check for absence
  @return the filter
  @mongodb.driver.manual reference/operator/query/exists $exists