Skew timers for ONT stats collection

As per VOL-119, VOLTHA supports PM collection configuration, including the frequency of collection and the metrics to be collected.

If PM collection is configured for a PM record type, then the OpenOLT adapter collects the measurements on the configured interval, and publishes the record to the Kafka bus.

In addition to the record information defined in VOL-930, the autonomous record shall include:

• Specified Interval duration (15-minute, daily)
• Validity of collection (valid, invalid, unavailable)

Regarding the timing of the measurement interval in the OLU or ONT hardware adapter -
• Hardware may not have a timer to trigger the interval adn push the collected measurement to OpenOLT
• In this case, the OpenOLT adapter will need to trigger collections at measurement intervals, such as by using NTP - ensure that this trigger is skewed across all OLTs – ie don't trigger them all at the same time
• Need to document exception case for loss of management visibility, including ability for hardware or OMCI to store historical intervals (or not) upon loss of management access

• Need to document possible variation in the duration of the measurement interval from the specified duration (eg, 15 minute or daily) due to the virtualization of collection by NTP trigger and polling