clientv3/op.go
func WithFromKey() OpOption {
	return func(op *Op) {
		if len(op.key) == 0 {
			op.key = []byte{0}
		}
		op.end = []byte("\x00")
	}
}
func WithFromKey() OpOption {
	return func(op *Op) {
		if len(op.key) == 0 {
			op.key = []byte{0}
		}
		op.end = []byte("\x00")
	}
}
// WithFromKey specifies the range of 'Get', 'Delete', 'Watch' requests
// to be equal or greater than the key in the argument.