chore: improve error message in _metadata.py (#1652)

* improve error message

* log last error response

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* update secret

* update test case to make sure failure reason is included

* update test

* update secret

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: harkamaljot

google/auth/compute_engine/_metadata.py

@@ -201,7 +201,7 @@ def get(
     url = _helpers.update_query(base_url, query_params)
 
     backoff = ExponentialBackoff(total_attempts=retry_count)
-
+    failure_reason = None
     for attempt in backoff:
         try:
             response = request(url=url, method="GET", headers=headers_to_use)
@@ -213,6 +213,11 @@ def get(
                     retry_count,
                     response.status,
                 )
+                failure_reason = (
+                    response.data.decode("utf-8")
+                    if hasattr(response.data, "decode")
+                    else response.data
+                )
                 continue
             else:
                 break
@@ -225,10 +230,13 @@ def get(
                 retry_count,
                 e,
             )
+            failure_reason = e
     else:
         raise exceptions.TransportError(
             "Failed to retrieve {} from the Google Compute Engine "
-            "metadata service. Compute Engine Metadata server unavailable".format(url)
+            "metadata service. Compute Engine Metadata server unavailable due to {}".format(
+                url, failure_reason
+            )
         )
 
     content = _helpers.from_bytes(response.data)

system_tests/secrets.tar.enc

@@ -344,12 +344,32 @@ def test_get_return_none_for_not_found_error():
 @mock.patch("time.sleep", return_value=None)
 def test_get_failure_connection_failed(mock_sleep):
     request = make_request("")
-    request.side_effect = exceptions.TransportError()
+    request.side_effect = exceptions.TransportError("failure message")
 
     with pytest.raises(exceptions.TransportError) as excinfo:
         _metadata.get(request, PATH)
 
-    assert excinfo.match(r"Compute Engine Metadata server unavailable")
+    assert excinfo.match(
+        r"Compute Engine Metadata server unavailable due to failure message"
+    )
+
+    request.assert_called_with(
+        method="GET",
+        url=_metadata._METADATA_ROOT + PATH,
+        headers=_metadata._METADATA_HEADERS,
+    )
+    assert request.call_count == 5
+
+
+def test_get_too_many_requests_retryable_error_failure():
+    request = make_request("too many requests", status=http_client.TOO_MANY_REQUESTS)
+
+    with pytest.raises(exceptions.TransportError) as excinfo:
+        _metadata.get(request, PATH)
+
+    assert excinfo.match(
+        r"Compute Engine Metadata server unavailable due to too many requests"
+    )
 
     request.assert_called_with(
         method="GET",