fix: refresh token prior to metadata exchange (#351)

In heavy usage, there can be cases where the client does not refresh the
token but then the token expires prior to the metadata exchange. This
commit ensures that the token is always fresh before proceeding to the
metadata exchange.

Fixes #346

google/cloud/alloydb/connector/connector.py

@@ -23,7 +23,9 @@
 from typing import Any, Dict, Optional, Type, TYPE_CHECKING, Union
 
 from google.auth import default
+from google.auth.credentials import TokenState
 from google.auth.credentials import with_scopes_if_required
+from google.auth.transport import requests
 
 from google.cloud.alloydb.connector.client import AlloyDBClient
 from google.cloud.alloydb.connector.enums import IPTypes
@@ -258,6 +260,10 @@ def metadata_exchange(
         if enable_iam_auth:
             auth_type = connectorspb.MetadataExchangeRequest.AUTO_IAM
 
+        # Ensure the credentials are in fact valid before proceeding.
+        if not self._credentials.token_state == TokenState.FRESH:
+            self._credentials.refresh(requests.Request())
+
         # form metadata exchange request
         req = connectorspb.MetadataExchangeRequest(
             user_agent=f"{self._client._user_agent}",  # type: ignore