This commit introduces two fields to the execution stats explain output of $group in the classic engine:

• spillFileSizeBytes, which reports the size of the spill file written to disk
• numBytesSpilledEstimate, which reports an estimate of the size of $group data evicted from memory as part of spilling

Note that these are not the same quantity, typically, spillFileSizeBytes will be less than numBytesSpilledEstimate because the buffer will be compressed before being written to disk.

Description of Linked Ticket

$group's executionStats should expose a stats "spillFileSizeBytes". This stat should report the maximum files size bytes that was written to disk in bytes. This stat will help in better planning/recommendation on disk usage and maybe I/O capacity planning.