来自readme：
Making your instance a Balancer is pretty simple. Just export the following environment variable.
export CLUSTER_BALANCER_URL=https://subdomain1.domainname.com
export CLUSTER_BALANCER_URL=https://subdomain2.domainname.com
我可以使用ip吗？
server1: export CLUSTER_BALANCER_URL=https://server1_ip
server2: export CLUSTER_BALANCER_URL=https://server2_ip