cookie_session Authenticator not working with gRPC middleware

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Describe the bug

The gRPC middleware httpRequest method transforms selected grpc metadata keys to http headers. The only metadata items translated are "authorization" and those with keys beginning with "x-" (see https://github.com/ory/oathkeeper/blob/master/middleware/grpc_middleware.go#L33-L41).

This means the Cookie header is not set and the cookie_session authenticator fails.

Reproducing the bug

Create a simple Go/gRPC app that uses the OathKeeper UnaryServerInterceptor
Configure the cookie_session authenticator (see below)
Add a "cookie" gRPC metadata item (using a valid Ory session cookie)
Make a gRPC request (Postman gRPC request)

Relevant log output

No response

Relevant configuration

config.yaml:

serve:
  proxy:
    port: 4455
  api:
    port: 4456

access_rules:
  repositories:
    - file://rules.yaml

authenticators:
  cookie_session:
    enabled: true
    config:
      check_session_url: https://<project-slug>.projects.oryapis.com/sessions
      preserve_path: true
      force_method: GET

authorizers:
  allow:
    enabled: true

rules.yaml:

- id: some-id
  match:
    authority: localhost:50001
    full_method: hello.helloService/Hello
  authenticators:
    - handler: cookie_session

Version

v0.40.1-0.20220923195049-ae7b65b54090 (latest commit)

On which operating system are you observing this issue?

macOS

In which environment are you deploying?

No response

Additional Context

Making the following change to https://github.com/ory/oathkeeper/blob/master/middleware/grpc_middleware.go#L36

// if k == "authorization" || strings.HasPrefix(k, "x-") {
if k == "authorization" || k == "cookie" || strings.HasPrefix(k, "x-") {

leads to a successful request

ory / oathkeeper

cookie_session Authenticator not working with gRPC middleware #1023