statistics: rename `RegionStats` in hot statistics #22171

Job	Run time
chunks (1, Unit Test(1))	5m 8s
chunks (2, Unit Test(2))	3m 59s
chunks (3, Unit Test(3))	5m 28s
chunks (4, Tests(1))	5m 43s
chunks (5, Tests(2))	5m 14s
chunks (6, Tools Test)	6m 1s
chunks (7, Client Integration Test)	5m 8s
chunks (8, TSO Integration Test)	4m 41s
chunks (9, MicroService Integration(!TSO))	4m 54s
chunks (10, MicroService Integration(TSO))	5m 57s
report-coverage	34s
	52m 47s

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