@Nullable
    @Override
    public ResponseConverterFunction createResponseConverterFunction(Type returnType) {
        if (isSupportedType(returnType)) {
            return new ProtobufResponseConverterFunction();
        }
        return null;
    }
Provides a {@link ProtobufResponseConverterFunction} to annotated services.
protobuf/src/main/java/com/linecorp/armeria/server/protobuf/ProtobufResponseConverterFunctionProvider.java