stats: Improved sequencing documentation for server-side stats events and added tests. #13398

Job	Run time
vet-proto	21s
tests (vet, 1.22)	1m 41s
tests (extras, 1.23)	2m 49s
tests (tests, 1.23)	4m 9s
tests (tests, 1.23, -race)	6m 2s
tests (tests, 1.23, 386)	4m 51s
tests (tests, 1.23, arm64)	6m 37s
tests (tests, 1.22)	8m 53s
tests (tests, 1.23, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=true)	4m 55s
	40m 18s

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