Number of NAKs this source transport has not processed because it met its
retransmission rate limit (LBT-RM, LBT-RU).

- n (Int32)
- Statistics set index

Number of NAKs this source did not process because doing so would
exceed its retransmit rate limit.

This rate limit is set by configuration option
transport_lbtrm_retransmit_rate_limit (default 5Mbps).
For each of these NAKs, the source transport immediately sends an NFC
rx_delay, then queues the retransmission for a later send within the rate
limit. If this count is high, one or more crybaby receiver transports may be
clogging the source transport's retransmit queue.
This method returns a value from the nth set of statistics
encapsulated within this object.