public static <TExpression> BsonField avg(final String fieldName, final TExpression expression) {
        return accumulator("$avg", fieldName, expression);
    }
public static < TExpression > BsonField avg ( final String fieldName , final TExpression expression ) { return accumulator ( "$avg" , fieldName , expression ) ; }
Gets a field name for a $group operation representing the average of the values of the given expression when applied to all
members 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/avg/ $avg