code: func (tracker *ImportTracker) AddNullable() {
  	tracker.AddType(&types.Type{
  		Kind: types.Protobuf,
  		Name: types.Name{
  			Name:    "nullable",
  			Package: "gogoproto",
  			Path:    "github.com/gogo/protobuf/gogoproto/gogo.proto",
  		},
  	})
  }
docs: // AddNullable ensures that support for the nullable Gogo-protobuf extension is added.