ESPv2 with Google Cloud Endpoints using gRPC returning "The current request is not defined"

Question

I'm trying to get ESPv2 with Cloud Endpoints working to proxy to a service using gRPC. (We've been already extensively using ESPv2 with OpenApi, but this is the first gRPC service we're trying to use it with.)

Here is what I did:
I have a proto contract like this:

syntax = "proto3";

package postsaleapi.v1;

service PostSaleService {
  rpc GetAllProducts (GetAllProductsRequest) returns (GetAllProductsResponse);
}

message GetAllProductsRequest {
  string order_number = 1;
}

message GetAllProductsResponse {
  ...
}

This post_sale_service.proto file is under the path /postsaleapi/v1/ in the repository folder containing our contracts.

Then I've created the compiled proto descriptor with protoc from the root folder of the repo:

protoc --proto_path=. --include_imports --include_source_info --descriptor_set_out=api_descriptor.pb .\postsaleapi\v1\post_sale_service.proto

This seems to have worked correctly, it created the api_descriptor.pb file, and if I do

protoc --decode_raw < api_descriptor.pb

It prints out all the details of the contract.

Then I created the service config file serviceConfig.yaml:

type: google.api.Service
name: postsaleapi-esp.stg.ourcompany.com
apis:
  - name: postsaleapi.v1.PostSaleService
usage:
  rules:
  - selector: postsaleapi.v1.PostSaleService.GetAllProducts
    allow_unregistered_calls: true

(Eventually I'd like to use authentication, but for now I just want to get the ESP working without any auth.)

And created the service with gcloud:

gcloud endpoints services deploy api_descriptor.pb serviceConfig.yaml

It is successfully created.

And I've deployed the ESP, the container has the following setup:

Containers:
  postsaleapi-esp:
    Container ID:  docker://afe7fa3e01fb2575df2a6779c839fdbbf45986f6d54ce90bf2ed9c895382df9b
    Image:         gcr.io/endpoints-release/endpoints-runtime:2.32.0
    Image ID:      docker-pullable://gcr.io/endpoints-release/endpoints-runtime@sha256:dfe8e931e26f7894dc0ba1eecec51423e8f9cec7fe1d5957e717a104423bec57
    Port:          8443/TCP
    Host Port:     0/TCP
    Args:
      --listener_port=8443
      --backend=https://postsaleapi.stg.internal.ourcompany.io:443
      --service=postsaleapi-esp.stg.ourcompany.com
      --service_account_key=/gcp-service-account/service-account-key.json
      --ssl_server_cert_path=/etc/envoy/ssl
      --backend_dns_lookup_family=v4only
      --enable_debug
      --http_request_timeout_s=60
      --rollout_strategy=managed

The underlying gRPC service is accessible at https://postsaleapi.stg.internal.ourcompany.io, and the standalone ESP is being accessed via a LoadBalancer Service.

If I try to call the GetAllProducts gRPC endpoint, I'm getting back this response:

{
  "error": "12 UNIMPLEMENTED: {\"code\":404,\"message\":\"The current request is not defined by this API.\"}"
}

If I check the logs of the ESP container, it prints these lines (among others) when I try testing the endpoint:

D0426 10:36:28.074 44 src/envoy/http/service_control/handler_impl.cc:114] [44][filter]No route entry
D0426 10:36:28.074 44 src/envoy/http/service_control/handler_impl.cc:92] [44][filter]No operation found
D0426 10:36:28.074 44 src/envoy/http/service_control/handler_impl.cc:95] [44][filter]Use non matched requirement.
D0426 10:36:28.074 44 external/envoy/source/common/http/filter_manager.cc:947] [44][http][C4][S10619230553122087859] Sending local reply with details direct_response
D0426 10:36:28.075 44 src/envoy/http/grpc_metadata_scrubber/filter.cc:29] [44][filter]Filter::encodeHeaders is called.
D0426 10:36:28.075 44 external/envoy/source/common/http/conn_manager_impl.cc:1467] [44][http][C4][S10619230553122087859] encoding headers via codec (end_stream=true):\n\':status\', \'200\'\n\'content-type\', \'application/grpc\'\n\'grpc-status\', \'12\'\n\'grpc-message\', \'{\"code\":404,\"message\":\"The current request is not defined by this API.\"}\'\n\'date\', \'Tue, 26 Apr 2022 10:36:28 GMT\'\n\'server\', \'envoy\'\n\'x-envoy-decorator-operation\', \'ingress UnknownOperationName\'

Can anybody tell what might be missing, or how I could troubleshoot further what is going wrong?