@Nullable
    @Override
    public RequestConverterFunction createRequestConverterFunction(Type requestType,
                                                                   RequestConverterFunction requestConverter) {
        final ResultType resultType = toResultType(requestType);
        if (resultType != ResultType.UNKNOWN) {
            return new ProtobufRequestConverterFunction(resultType);
        } else {
            return null;
        }
    }
Provides a {@link ProtobufRequestConverterFunction} to annotated services.
protobuf/src/main/java/com/linecorp/armeria/server/protobuf/ProtobufRequestConverterFunctionProvider.java