The index for the Measurement Interval within this
PM session.
Measurement Interval indexes are assigned sequentially by
the SNMP Agent. The first Measurement Interval that occurs after
the session is started is assigned index 1.
Referential integrity is necessary, i.e., the index needs to be
persistent upon a reboot or restart of a device. The index
is never reused while this session is active until it wraps to zero.
The index value keeps increasing up to that time.