code: public static <TExpression> BsonField first(final String fieldName, final TExpression expression) {
          return accumulator("$first", fieldName, expression);
      }
docs: Gets a field name for a $group operation representing the value of the given expression when applied to the first member of
  the group.
  
  @param fieldName the field name
  @param expression the expression
  @param <TExpression> the expression type
  @return the field
  @mongodb.driver.manual reference/operator/aggregation/first/ $first