SPM-3701 added four spilling metrics in all stages that spill

• spills // The number of times the stage has spilled
• spilledBytes // The amount in bytes of main memory that has been spilled
• spilledRecords // The number of records that have been spilled
• spilledDataStorageSize // The amount in bytes that spilling has taken up on disk

The stages report those metrics in their explain output but there is no accumulated result for the whole query. It would be nice to report the total of the above metrics for the whole query in the explain output.