ServiceAccountJwtAccessCredentials example?

[grandpaslab]

I'm attempting to use ServiceAccountJwtAccessCredentials to connect to a 3rd-party google cloud endpoint. AFAICT there's no example for using this class, and I haven't been able to get it to work. I've got a python example working, so I know the audience and whatnot are correct. I've cobbled together some code based on the ServiceAccountCredentials example, but I can't tell what I'm doing wrong. The error I'm getting from the API is 401: "jwt is missing".

`

$path = 'cred.json';
$sa = new ServiceAccountJwtAccessCredentials($path);

$metadata = $sa->updateMetadata(
    [
       'issuer' => 'ham@sandwich.iam.gserviceaccount.com',
       'client_email' => 'ham@sandwich.iam.gserviceaccount.com',
       'audience' => 'https://ham-sandwich.a.run.app'
    ],
    "https://accounts.google.com/o/oauth2/auth" # auth_uri
);

$middleware = new AuthTokenMiddleware($sa); 
$stack = HandlerStack::create(); 
$stack->push($middleware);
$client = new Client([ 
    'handler' => $stack, 
    'base_uri' => $BASE_URI, 
    'auth' => 'google_auth'
]);

$res = $client->request('POST', $SERVICE_URI, [
    ['body' => json_encode($quote_data)]
]);

`

Can an example for using this class be added to the docs?