[protobuf] C++ : Access fields extentions from a FieldOptions in DynamicMessage?

[Nishar Nalakathu]

extend google.protobuf.FieldOptions {
    optional string index1 = 10000;
    optional string index2 = 100001;
}
message order {
    required string id = 1
        [(index1) = "map"];
    required string name = 2
        [(index2) = "multimap" ];
}

I use the dynamic message to create and access the message as follows,

        const google::protobuf::FileDescriptor * 
fd=i->Import("order.proto");
        google::protobuf::DynamicMessageFactory factory;
        const google::protobuf::Descriptor * md= 
fd->FindMessageTypeByName("order");
        const google::protobuf::Message* no_prototype = 
factory.GetPrototype(md);
        google::protobuf::Message* msg = no_prototype->New();
        const pb::FieldDescriptor *fd = md->field(i);
        const pb::FieldOptions & fo = fd->options();

That is working fine, but how can I access the fields extensions from 
FieldOptions for a dynamic message ? 
Actually, I want to set few static attribute for some fields and retrieve 
those at runtime as same as getting the FieldDescriptor info from message 
descriptor. 

extend google.protobuf.FieldOptions {
    optional string index1 = 10000;
    optional string index2 = 100001;
}

message order {
    required string id = 1
        [(index1) = "map"];
    required string name = 2
        [(index2) = "multimap" ];
}

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