Work done under SERVER-51026 recently extended the query execution systems runtime stats to track the individual number of times that a sort stage spills to disk. However, explain currently only reports whether a SORT PlanStage used disk with a boolean; if disk was used, we don't indicate the number of times that we spilled sorted data to disk. We should take advantage of the work done in SERVER-51026 and report this "spills" metric for the SORT stage.

Additionally, it would be good to report this as a runtime stat for the $sort DocumentSource. We could also make a similar extension and report the number of spills for a $group. This could build on the work done in SERVER-48380 and SERVER-49027 to add additional runtime stats for $sort and $group to agg explain output.