Allow the "step" (or interval) to be referenced from a Measurement API expression

Using the Measurements API we would like to be able to get the total number of errors on an interface in a given time range. The number of errors are collected and stored counters which means that the the Measurement API will return the rate of errors (errors per second) when generating the result sets.

In order to get the total number of errors we need to multiply the rate by the step, but the value of the step is not available in the expression context.