OpenAPI Generation Issue: Empty Specification for Protocol Buffer Service

Start at the protoc-gen-openapi plugin entry point and reproduce the command with the supplied petstore.proto. Compare the generated openapi.yaml with the service, request, and response definitions; done means the output contains the expected paths and schemas instead of empty objects.

Issue Summary:
I am experiencing a problem with the Gnostic library and the protoc-gen-openapi plugin when generating an OpenAPI specification from a Protocol Buffer service. The generated OpenAPI specification is empty and does not reflect the contents of the Protocol Buffer service definition.

Steps to Reproduce:

1. Create a Protocol Buffer service definition, like the one in Attachment 1.
2. Use the following command to generate the OpenAPI specification:
   protoc petstore.proto -I=. --openapi_out=.
3. Examine the generated OpenAPI YAML specification (Attachment 2), which is expected to describe the Protocol Buffer service.

Expected Behavior:
The generated OpenAPI specification should accurately represent the Protocol Buffer service and its endpoints, including any request and response message types.

Actual Behavior:
The generated OpenAPI specification is empty, with no paths, components, or schemas, and it does not reflect the structure of the Protocol Buffer service.

Attachments:

1. Input Protobuf service (petstore.proto)

syntax = "proto3";

package swaggerpetstore_openapi3_1;

option go_package = "swaggerpetstore/openapi3_1";

message GetPetByIdRequest {
    // ID of pet to return
    int64 petId = 1;
}

message Pet {
  int64 id = 1;
  string name = 2;
  Category category = 3;
  repeated string photoUrls = 4;
  repeated Tag tags = 5;
  string status = 6;

  message Category {
    int64 id = 1;
    string name = 2;
  }

  message Tag {
    int64 id = 1;
    string name = 2;
  }
}

service SwaggerPetstoreOpenAPI31Service {
    // Find pet by ID
    //
    // Returns a single pet
    rpc GetPetById(GetPetByIdRequest) returns (Pet) {}
}

2. Output OpenAPI definition (openapi.yaml)

# Generated with protoc-gen-openapi
# https://github.com/google/gnostic/tree/master/cmd/protoc-gen-openapi

openapi: 3.0.3
info:
    title: ""
    version: 0.0.1
paths: {}
components:
    schemas: {}

Additional Information:
Go Version: go1.21.4 windows/amd64
Gnostic Library Version: installed with go install github.com/google/gnostic@latest so I guess v0.7.0 (?)
Protocol Buffer Compiler Version: libprotoc 25.0 (protoc-25.0-win64)
Operating System: Windows 11 Enterprise build 22631