Currently, only the $cursor stage reports any interesting execution statistics when explaning an aggregation at verbosity level "executionStats". Because of the complexity of $exchange, it would be interesting to expose more runtime stats.

Some ideas of what might be interesting off the top of my head:

• number of total calls to DocumentSourceExchange::getNext()
• number of total yields
• number of times each consumer blocks on empty buffer
• number of times each consumer thread fills the buffers
• the number of times buffer N is filled to capacity