TopicAdminClient UnauthenticatedException When Settings Endpoints

[benshriki]

Hi,
I've found an issue when settings TopicAdminClient with endpoint.

Current behavior:

• When i try to set TopicAdminClient with credentials out of a json(String) and endpoint different then the global "pubsub.googleapis.com" end point i get an UnauthenticatedException.
• When i use ADC authentication and set the environment variable GOOGLE_APPLICATION_CREDENTIALS to point to a json file with the same content i can set to all endpoints with no errors.
• When i try to set TopicAdminClient with credentials out of a json(String) and use the global endpoint pubsub.googleapis.com" i get no errors.

Expected behavior:
To be able to use both authentication ways and set any endpoint without encountering any error.

Example:

...
ServiceAccountCredentials cred = ServiceAccountCredentials
              .fromStream(new ByteArrayInputStream(jsonCredentials.getBytes()));  // json as String

CredentialsProvider credentialsProvider = FixedCredentialsProvider.create(cred); 

TopicAdminSettings topicAdminSettings = TopicAdminSettings.newBuilder()
                                 .setCredentialsProvider(credentialsProvider)   // credentials out of given json file
                                 .setEndpoint(endpoint)                         // endpoint different then global endpoint
                                 .build();

TopicAdminClient topicAdminClient = TopicAdminClient.create(topicAdminSettings);

topicAdminClient.getTopic(someTopic);

the code above produce the following exception:
com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.'

[kamalaboulhosn]

This issue should be in the Java library repo, so I have created it there: googleapis/java-pubsub#1480.