expression

The math expression to be performed on the returned data, if this object is performing a math expression. This expression can use the `Id` of the other metrics to refer to those metrics, and can also use the `Id` of other expressions to use the result of those expressions. For more information about metric math expressions, see Metric Math Syntax and Functions in the User Guide. Within each MetricDataQuery object, you must specify either `Expression` or `MetricStat` but not both.