FieldDescriptor fieldDescriptor =
descriptor.findFieldByName("optional_nested_message");
FieldDescriptor foreignFieldDescriptor =
descriptor.findFieldByName("optional_foreign_message");
FieldDescriptor importFieldDescriptor =
descriptor.findFieldByName("optional_import_message");
// Mutate the message with new field builder
// Mutate nested message
TestAllTypes.Builder builder1 = TestAllTypes.newBuilder();
Message.Builder fieldBuilder1 = builder1.newBuilderForField(fieldDescriptor)