Issue description

服务发现使用为DNS后，添加路由时，服务名称一定要带端口号吗，测试发现不带端口都被转发到80端口上去了
config.yaml:
discovery:
dns:
servers:
- "127.0.0.1:8600" # use the real address of your dns server
curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -i -d '
{
"uri": "/gatewayd/",
"upstream": {
"service_name": "gateway.service.consul"
"type": "roundrobin",
"discovery_type": "dns"
},
"plugins": {
"proxy-rewrite" : {
"regex_uri": ["^/gatewayd/(.)", "/${1}"]
}
}
}'

而实际环境中一个服务对应多个节点，端口都是随机不一致的
不能自动获取consul 上注册上去的端口么

Environment

• apisix version (cmd: apisix version): 2.9
• OS (cmd: uname -a): centos 7 Linux localhost.localdomain 3.10.0-1160.el7.x86_64 change: added doc of how to load plugin. #1 SMP Mon Oct 19 16:18:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):openresty/1.19.9.1
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API):3.5.0
• apisix-dashboard version, if have:2.7
• consul version :V1.10.2