The number of previous 15-minute intervals in the
interval table for which data was collected. Given
that <n> is the maximum # of intervals supported.
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.