Google has a repository on GitHub to do exactly that. It contains examples for OpenAPI v3 (and v2) on how the equivalent protobuf code should look like.
Here is a medium article that explains end to end gRPC + HTTP (using protobuf and swagger respectively) server implementation and documentation, in a step by step fashion. It is an OpenAPI v2 example, but similar concepts apply to OpenAPI v3.
There are other repositories that can do the same, but I would recommend to use the source code that is actually coming from the very source. :)
Hope this helps!