rpc错误：代码=不可用 描述=服务器当前正在关闭

解决方案

1. 80% 成功率 Implement retry with backoff to wait for the server to restart.

   for i := 0; i < 5; i++ {
       resp, err := client.SomeRPC(ctx, req)
       if err == nil { return resp }
       if status.Code(err) != codes.Unavailable { return err }
       time.Sleep(time.Duration(1<<i) * time.Second)
   }

2. 85% 成功率 Use a load balancer to redirect requests to healthy instances.

   Configure a load balancer that detects server health and routes traffic away from shutting down instances.