The number of previous near-end performance data 15-minute
intervals for which data was collected. The value will be <n>
unless the measurement was (re-)started within the last (<n>*15)
minutes, in which case the value will be the number of complete
15-minute intervals for which the agent has at least some data.
In certain cases (e.g., in the case where the agent is a proxy)
it is possible that some intervals are unavailable. In this
case, this interval is the maximum interval number for which
data is available.