Nacos service registry cannot work because of ServerTags matching failure

[jthann]

Describe the bug
After upgrading Easegress from v1.x to v2.0.0， Nacos service registry(maybe including others service registry) cannot work. Because in following method from pkg/filters/proxy/pool.go:309.

The Tags attribute in ServiceInstanceSpec object and ServerTags in ServerPoolSpec object are always nil, this will lead to servers is always empty and proxy filter return 503 status code.

func (sp *ServerPool) useService(instances map[string]*serviceregistry.ServiceInstanceSpec) {
	servers := make([]*Server, 0)
	for _, instance := range instances {
		for _, tag := range sp.spec.ServerTags {
			if stringtool.StrInSlice(tag, instance.Tags) {
				servers = append(servers, &Server{
					URL:    instance.URL(),
					Tags:   instance.Tags,
					Weight: instance.Weight,
				})
				break
			}
		}
	}
   //......
}