code: public static <TExpression> BsonField push(final String fieldName, final TExpression expression) {
          return accumulator("$push", fieldName, expression);
      }
docs: Gets a field name for a $group operation representing an array of all values that results from applying an expression to each
  document in a group of documents that share the same group by key.
  
  @param fieldName the field name
  @param expression the expression
  @param <TExpression> the expression type
  @return the field
  @mongodb.driver.manual reference/operator/aggregation/push/ $push