private static class GenerateProtoPairs extends DoFn<String, Pair<String, Pair<String, Person>>> {
@Override
public void process(String input, Emitter<Pair<String, Pair<String, Person>>> emitter) {
String[] fields = input.split(",");
String key = fields[0];
Builder b = Person.newBuilder().setFirst("first"+key).setLast("last"+key);
emitter.emit(Pair.of(fields[0], Pair.of(fields[1], b.build())));
}