SERVER-21784 recently added execution stats to the agg execution layer, and exposed them via "executionStats" or "allPlansExecution" explain verbosities. This ticket, however, added only nReturned and executionTimeMillis for every stage. There are more stats that we can expose which will be useful for debugging and performance investigations.
One suggestion from Alex Bevilacqua is to expose the amount of data processed by $sort or $group. We have such stats for sorts executed in the PlanStage layer, but not for sorts executed in the DocumentSource layer. The $sort stage would report a totalDataSizeSortedBytesEstimate metric, and the $group stage would report totalOutputDataSizeBytes.
Another idea that we could consider implementing at the same time is to report usedDisk:true when either a $sort or a $group spills to disk at runtime.