client/http: expose approximate_kv_size in RegionInfo #28477

Job	Run time
chunks (4, Tests(1))	3m 21s
chunks (6, Tools Test)	3m 22s
chunks (8, TSO Integration Test)	3m 21s
chunks (1, Unit Test(1))	3m 20s
chunks (10, Microservice Integration(TSO))	3m 22s
chunks (5, Tests(2))	3m 20s
chunks (9, Microservice Integration(!TSO))	3m 21s
chunks (2, Unit Test(2))	3m 21s
chunks (3, Unit Test(3))	3m 20s
chunks (7, Client Integration Test)	3m 21s
report-coverage	9s
	33m 38s

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