using the auth library with grpc fails with Error: 7 PERMISSION_DENIED

^{Originally posted by **odedpeer** June 30, 2025}
### Please make sure you have searched for information in the following guides.

- [x] Search the issues already opened: https://github.com/GoogleCloudPlatform/google-cloud-node/issues
- [x] Search StackOverflow: http://stackoverflow.com/questions/tagged/google-cloud-platform+node.js
- [x] Check our Troubleshooting guide: https://github.com/googleapis/google-cloud-node/blob/main/docs/troubleshooting.md
- [x] Check our FAQ: https://github.com/googleapis/google-cloud-node/blob/main/docs/faq.md
- [x] Check our libraries HOW-TO: https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md
- [x] Check out our authentication guide: https://github.com/googleapis/google-auth-library-nodejs
- [x] Check out handwritten samples for many of our APIs: https://github.com/GoogleCloudPlatform/nodejs-docs-samples

### A screenshot that you have tested with "Try this API".

this works in API Explorer.
this doesn't work with the service account that requires impersonation, which I can't test in API explorer.


### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://gist.github.com/odedpeer/0aa4ca8b2efc928cb19806c5a129a5a4

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

* create a node project
* yarn add @googleapis/iam@28.0.1
* yarn add @google-cloud/logging@11.2.0
* create a service account with domain wide delegation as explained in https://developers.google.com/identity/protocols/oauth2/service-account#delegatingauthority
* set the service account email address and path to credentials file in the javascript file
* run the program

### A clear and concise description of what the bug is, and what you expected to happen.

I am working on upgrading our Google SDK packages using the latest packages:
```
"@googleapis/iam": "28.0.1",
"google-auth-library": "10.1.0",
"@google-cloud/logging": "11.2.0"
```

when I invoke the grpc call `Logging.getEntries()` from the `@google-cloud/logging package` it results in error
`Error: 7 PERMISSION_DENIED: Method doesn't allow unregistered callers (callers without established identity). Please use API Key or other form of API consumer identity to call this API.`

I expect to list all the log entries.

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

I was able to track this down to the following regression bug:

https://github.com/googleapis/google-auth-library-nodejs/pull/1938 changed the `getRequestMetadataAsync` function from returning a JSON to returning an object
https://github.com/googleapis/google-auth-library-nodejs/pull/1938/files#diff-320fe75112224dd3e6d983b88d752cd3b6ab866ef7fcb8723cd30975b6a5f869R955

`@grpc` module converts the headers to a Metadata object by using `Object.keys` to fill the metadata object.
this used to work with JSON objects, but it does not work with `Headers` since the keys are not object properties.
https://github.com/grpc/grpc-node/blob/master/packages/grpc-js/src/call-credentials.ts#L130C42-L131C29