code: public static <TExpression> BsonField stdDevPop(final String fieldName, final TExpression expression) {
          return accumulator("$stdDevPop", fieldName, expression);
      }
docs: Gets a field name for a $group operation representing the sample standard deviation of the values of the given expression
  when applied to all members of the group.
  
  <p>Use if the values encompass the entire population of data you want to represent and do not wish to generalize about
  a larger population.</p>
  
  @param fieldName the field name
  @param expression the expression
  @param <TExpression> the expression type
  @return the field
  @mongodb.driver.manual reference/operator/aggregation/stdDevPop/ $stdDevPop
  @mongodb.server.release 3.2
  @since 3.2