ringhash: spread out endpoints in the ring

[easwars]

Currently, when creating the ring, we assign the same hash to all entries corresponding to one address. This means that the ring will have addresses in a contiguous fashion. https://github.com/grpc/grpc-go/blob/28de4866ce7440b675662abbdd5c43b476bd4dae/xds/internal/balancer/ringhash/ring.go#L98

Spreading out instances of the same address in the ring seems better, because when subchannels are unreachable, the picker goes through the ring in order, so it's better to not have the same address many times in a row. This is what c-core does, and we should as well.

[easwars]

Actually, looks like we are already spreading the instances of the same address across the ring.

for float64(idx) < targetIdx {
	h := xxhash.Sum64String(scw.sc.addr + strconv.Itoa(len(items)))
	items = append(items, &ringEntry{idx: idx, hash: h, sc: scw.sc})
	idx++
}

items is pre-allocated with capacity. But len(items) keeps changing with each iteration of the above loop as we append more entries to the slice.

So, nothing needs to be done here.