misc: Add toString() to OutputBuffer stats #15109

Job	Run time
Build	48m 51s
Build with Clang	0s
Spark Aggregate Fuzzer	15m 42s
Spark Fuzzer	15m 47s
Presto Fuzzer	30m 48s
Spark Bias Fuzzer	0s
Exchange Fuzzer	16m 21s
Presto Bias Fuzzer	0s
Aggregation Fuzzer with Presto as source of truth	18m 2s
Join Fuzzer	17m 42s
Window Fuzzer with Presto as source of truth	17m 47s
RowNumber Fuzzer	15m 36s
Signature Changes	4s
Biased Aggregation Fuzzer with Presto as source of truth	0s
Biased Expression Fuzzer with Only Added/Updated Functions and Presto as source of truth	0s
Writer Fuzzer with Presto as source of truth	17m 24s
	3h 34m 4s

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