All spilling stages should report 4 metrics to serverStatus
spills : number of times the stage spilled,
spilledBytes : number of spilled bytes,
spilledRecords : number of spilled records,
spilledDataStorageSize : size of spilled data on disk
From all stages that spill, Spool (classic) does not report any metrics in ServerStatus while Sort reports spillToDisk and spillToDiskBytes. We should fix those two stages to report the same metrics as the rest of the spilling stages.