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. I would be nice to report the total of the above metrics for the whole query in the explain output.