getIamPolicy for KMS KeyManagementServiceClient is erroring with Syntax Error: Unexpected token < in JSON at position 0 , when using fallback REST

1) Is this a client library issue or a product issue?
Yes it is a client library issue, the definition for `getIamPolicy` method in iam_services.json mentions `POST` call whether the actual call is `GET`

2) Did someone already solve this?
Could not find any solution or issues created for the same.

3) Do you have a support contract?
No

#### Environment details

- OS: WSL 2Ubuntu 20.04.4 LTS
- Node.js version: 18.14.2
- npm version: 9.5.0
- `google-gax` version: 3.6.0

#### Steps to reproduce

1. Call the following method for KMS
```typescript
import * as kms from "@google-cloud/kms";
import { google } from "@google-cloud/iap/build/protos/protos";
import GetIamPolicyRequest = google.iam.v1.GetIamPolicyRequest;

const keyManagementServiceClient = new kms.KeyManagementServiceClient({
credentials: {
client_email: "user@project-id.iam.gserviceaccount.com",
private_key:"very-long-private-key",
},
projectId: "project-id",
fallback: "rest",
});

const req = new GetIamPolicyRequest({
resource: "projects/project-id/locations/global/keyRings/qflow",
});
keyManagementServiceClient
.getIamPolicy(req)
.then(console.log)
.catch(console.error);

```
2. It is using the POST method to hit the API
Output (Before)

3. After making a change to the following [file ](https://github.com/googleapis/gax-
nodejs/blob/main/protos/iam_service.json#L43,L49)and replacing `post` with `get` issue seems to be resolved.

Changes


Output (After)

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!