code: public static Bson geoWithinCenter(final String fieldName, final double x, final double y, final double radius) {
          BsonDocument center = new BsonDocument("$center",
                                                        new BsonArray(Arrays.<BsonValue>asList(new BsonArray(asList(new BsonDouble(x),
                                                                new BsonDouble(y))),
                                                                new BsonDouble(radius))));
          return new OperatorFilter<BsonDocument>("$geoWithin", fieldName, center);
      }
docs: Creates a filter that matches all documents containing a field with grid coordinates data that exist entirely within the specified
  circle.
  
  @param fieldName the field name
  @param x         the x coordinate of the circle
  @param y         the y coordinate of the circle
  @param radius    the radius of the circle, as measured in the units used by the coordinate system
  @return the filter
  @mongodb.driver.manual reference/operator/query/geoWithin/ $geoWithin
  @mongodb.driver.manual reference/operator/query/center/#op._S_center $center
  @mongodb.server.release 2.4
  @since 3.1