protobuf：如何从FieldOptions解析描述符消息

I have a proto file below

syntax = "proto2";
package cmd;

import "google/protobuf/descriptor.proto";

message FlagDetail {
  required string name = 1;
  required string value = 2;
  required string shorthand = 3;
  required string usage = 4;
}

extend google.protobuf.FieldOptions {
  optional FlagDetail info = 1234;
}

message VersionFlags {
  optional bool client = 2 [ (info) = { name: "client" value: "false" shorthand: "c" usage: "Client version only (no server required)."}];
  optional bool short = 3  [ (info) = { name: "short" value: "false" shorthand: "baz" usage: "Print just the version number."}];
  optional string output = 4 [ (info) = { name: "output" value: "" shorthand: "o" usage: "One of 'yaml' or 'json'."}];
}

How to a get an empty message's default value of FlagDetail

some thing like this

var msg VersionFlags
md := ForMessage(&msg)  
o := md.Field[0].GetOptions()
o.GetFlagDetail.GetName()  //unfortunately, there's no method like this

BTW ForMessage() is from here : https://github.com/golang/protobuf/blob/master/descriptor/descriptor_test.go

here is my proto file https://gist.githubusercontent.com/shiywang/3d9f53fe253bb4195d65b3626442cb66/raw/89c286599a4103f67b80a62c87c69847497fa289/protofile

_, md := descriptor.ForMessage(msg.(descriptor.Message))
info, err := proto.GetExtension(md.GetOptions(), cmdproto.E_Cmd)
if err != nil {
    panic(err)
}
return info.(*cmdproto.CommandInfo)

kind of like this, but the problem I actually met was this https://github.com/golang/protobuf/issues/372