The number of previous 15-minute intervals
for which data was collected.
The value will be 96 unless the measurement was
(re-)started within the last 24 hours, 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.