robustness: change mixedVersionOption to use ChoiceWeight.

[siyuanfoundation]

followup of #17923

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[serathius]

That's less readable than previous solution, by weight options like traffic, I literally meant the weighted choice from robustness traffic.

etcd/tests/robustness/traffic/random.go

Lines 31 to 49 in 7a3ca53

	type choiceWeight[T any] struct {
	choice T
	weight int
	}
	func pickRandom[T any](choices []choiceWeight[T]) T {
	sum := 0
	for _, op := range choices {
	sum += op.weight
	}
	roll := rand.Int() % sum
	for _, op := range choices {
	if roll < op.weight {
	return op.choice
	}
	roll -= op.weight
	}
	panic("unexpected")
	}

Example of usage:

etcd/tests/robustness/traffic/etcd.go

Lines 36 to 48 in 7a3ca53

	requests: []choiceWeight[etcdRequestType]{
	{choice: Get, weight: 15},
	{choice: List, weight: 15},
	{choice: StaleGet, weight: 10},
	{choice: StaleList, weight: 10},
	{choice: Put, weight: 23},
	{choice: LargePut, weight: 2},
	{choice: Delete, weight: 5},
	{choice: MultiOpTxn, weight: 5},
	{choice: PutWithLease, weight: 5},
	{choice: LeaseRevoke, weight: 5},
	{choice: CompareAndSet, weight: 5},
	},

[siyuanfoundation]

done

[siyuanfoundation]

/retest