func WithRange(endKey string) OpOption {
	return func(op *Op) { op.end = []byte(endKey) }
}
func WithRange(endKey string) OpOption {
	return func(op *Op) { op.end = []byte(endKey) }
}
// WithRange specifies the range of 'Get', 'Delete', 'Watch' requests.
// For example, 'Get' requests with 'WithRange(end)' returns
// the keys in the range [key, end).
// endKey must be lexicographically greater than start key.