I've created a public project that automates the generation of OpenAPI 3 documentation for gRPC methods. It also extracts request/response samples from unit tests and includes them directly in the OpenAPI file.

The only current limitation is that it assumes each microservice contains its own .proto file — it doesn't support setups where proto files are centralized in a shared repository.

I hope this project can be helpful or inspiring (feel free to check out the README)
Good luck!