This counts how many times a usage record's counter value overflowed during the current aggregation period, a condition that forces the switch to close that record and open a new one, provided there's still space in the aggregated usage data store. A nonzero and growing nodeBillingUsageRecOvflWarnings during a single aggregation period suggests unusually high traffic volume per service, possibly indicating nodeBillingAggrPeriod should be shortened to keep individual records from overflowing so often.