Add example for how to use well known types with buf

[banool]

So in the buf model you use buf to build everything, including the file descriptor sets (usually). But how you access the well known types was a bit of a mystery to me until I did some digging. In short you need to add the protoc-wkt crate and then use the file descriptor set like this:

use protoc_wkt::google::protobuf::{FILE_DESCRIPTOR_SET as GOOGLE_PROTOBUF_FILE_DESCRIPTOR_SET};

fn myfunc() {
    let reflection_service = tonic_reflection::server::Builder::configure()
        // Note: It is critical that the file descriptor set is registered for every
        // file that the API proto depends on recursively. If you don't, compilation
        // will still succeed but reflection will fail at runtime.
        //
        // TODO: Add a test for this / something in build.rs, this is a big footgun.
        .register_encoded_file_descriptor_set(API_V2_FILE_DESCRIPTOR_SET)
        .register_encoded_file_descriptor_set(TRANSACTION_V1_TESTING_FILE_DESCRIPTOR_SET)
        .register_encoded_file_descriptor_set(UTIL_TIMESTAMP_FILE_DESCRIPTOR_SET)
        .register_encoded_file_descriptor_set(GOOGLE_API_V1_FILE_DESCRIPTOR_SET)
        .register_encoded_file_descriptor_set(GOOGLE_PROTOBUF_FILE_DESCRIPTOR_SET)
        .build()
        .context("Failed to build reflection service")?;
}

It'd be great to have an example that demonstrates how to use them. If I get some time any time soon I can take a crack at doing it myself 🤠

[jalaziz]

I'm trying to setup the reflection service and it sure has been a pain to figure out where to get all the file descriptors.

In fact, I'm not sure how to get GOOGLE_API_V1_FILE_DESCRIPTOR_SET? Do I have to use --include-imports? In our case, we also use the protoc-gen-openapiv2 annotations and we're seeing reflection errors due to that.

While I appreciate protoc-gen-prost's approach to generating the FILE_DESCRIPTOR_SET, the default tonic-build approach does seem to make things slightly easier.