feat: Add debug logging that can help with diagnosing auth lib. path (#…

…473)

Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Co-authored-by: Tres Seaver <tseaver@palladion.com>

google/auth/_default.py

@@ -155,10 +155,13 @@ def _get_gcloud_sdk_credentials():
     """Gets the credentials and project ID from the Cloud SDK."""
     from google.auth import _cloud_sdk
 
+    _LOGGER.debug("Checking Cloud SDK credentials as part of auth process...")
+
     # Check if application default credentials exist.
     credentials_filename = _cloud_sdk.get_application_default_credentials_path()
 
     if not os.path.isfile(credentials_filename):
+        _LOGGER.debug("Cloud SDK credentials not found on disk; not using them")
         return None, None
 
     credentials, project_id = load_credentials_from_file(credentials_filename)
@@ -174,6 +177,10 @@ def _get_explicit_environ_credentials():
     variable."""
     explicit_file = os.environ.get(environment_vars.CREDENTIALS)
 
+    _LOGGER.debug(
+        "Checking %s for explicit credentials as part of auth process...", explicit_file
+    )
+
     if explicit_file is not None:
         credentials, project_id = load_credentials_from_file(
             os.environ[environment_vars.CREDENTIALS]
@@ -190,15 +197,20 @@ def _get_gae_credentials():
     # While this library is normally bundled with app_engine, there are
     # some cases where it's not available, so we tolerate ImportError.
     try:
+        _LOGGER.debug("Checking for App Engine runtime as part of auth process...")
         import google.auth.app_engine as app_engine
     except ImportError:
+        _LOGGER.warning("Import of App Engine auth library failed.")
         return None, None
 
     try:
         credentials = app_engine.Credentials()
         project_id = app_engine.get_project_id()
         return credentials, project_id
     except EnvironmentError:
+        _LOGGER.debug(
+            "No App Engine library was found so cannot authentication via App Engine Identity Credentials."
+        )
         return None, None
 
 
@@ -215,6 +227,7 @@ def _get_gce_credentials(request=None):
         from google.auth import compute_engine
         from google.auth.compute_engine import _metadata
     except ImportError:
+        _LOGGER.warning("Import of Compute Engine auth library failed.")
         return None, None
 
     if request is None:
@@ -229,6 +242,9 @@ def _get_gce_credentials(request=None):
 
         return compute_engine.Credentials(), project_id
     else:
+        _LOGGER.warning(
+            "Authentication failed using Compute Engine authentication due to unavailable metadata server."
+        )
         return None, None