如何将Go地图序列化为protobuf

I'm following this tutorial and got to the part on serializing/marshaling Go structs into a protocol buffer. My struct has a map and I can't find any documentation on how to handle marshaling a map.

In the following I want to serialize Fields map[string]string:

Go struct:

type Note struct {
    ID     NoteID
    Fields map[string]string
}

protobuf schema:

package internal;

message Note {
    optional int64 ID = 1;
    optional map<string, string> Fields = 2;
}

Go marshal:

func MarshalNote(n *remember.Note) ([]byte, error) {
    return proto.Marshal(&Note{
        ID: proto.Int64(int64(n.ID))
        Fields: proto.???
    })
}

I have no idea what to do for the last line and anything I search for talks about mapping a field to a protobuf scheme, and not about mapping a map to a protobuf scheme.

protobuf is a well defined searlization format and one of the benefits of use it, it generates all the data structures for you(on your favorite language) just using the protobuf schema, e.i. you don't have to do a manual marshal or unmarshal