chore: make cleanup_cloud_functions test fixture more robust to flakiness (#720)

* chore: make `cleanup_cloud_functions` test fixture more robust to flakiness

In response to a recent test failure in load test session just because
this optional cleanup code failed.

```
____________ ERROR at setup of test_read_gbq_sql_large_results[1gb] ____________
[gw1] linux -- Python 3.12.0 /tmpfs/src/github/python-bigquery-dataframes/.nox/load/bin/python

...

tests/system/conftest.py:1119:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tests/system/utils.py:349: in delete_cloud_function
    operation = functions_client.delete_function(request=request)
.nox/load/lib/python3.12/site-packages/google/cloud/functions_v2/services/function_service/client.py:1499: in delete_function
    response = rpc(
.nox/load/lib/python3.12/site-packages/google/api_core/gapic_v1/method.py:131: in __call__
    return wrapped_func(*args, **kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

args = (name: "projects/bigframes-load-testing/locations/us-central1/functions/bigframes-ab103e98fa51cfc67a56d754b7e5068e-u8owkeqc"
,)
kwargs = {'metadata': [('x-goog-request-params', 'name=projects/bigframes-load-testing/locations/us-central1/functions/bigframe...6d754b7e5068e-u8owkeqc'), ('x-goog-api-client', 'bigframes/1.7.0 ibis/8.0.0 gl-python/3.12.0 grpc/1.64.0 gax/2.19.0')]}

    @functools.wraps(callable_)
    def error_remapped_callable(*args, **kwargs):
        try:
            return callable_(*args, **kwargs)
        except grpc.RpcError as exc:
>           raise exceptions.from_grpc_error(exc) from exc
E           google.api_core.exceptions.ServiceUnavailable: 503 502:Bad Gateway

.nox/load/lib/python3.12/site-packages/google/api_core/grpc_helpers.py:78: ServiceUnavailable
```

* include warning about failed cleanup

* 🦉 Updates from OwlBot post-processor

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

---------

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

Author: tswast

tests/system/conftest.py

@@ -20,6 +20,7 @@
 import textwrap
 import typing
 from typing import Dict, Generator, Optional
+import warnings
 
 import google.api_core.exceptions
 import google.cloud.bigquery as bigquery
@@ -1129,13 +1130,21 @@ def cleanup_cloud_functions(session, cloudfunctions_client, dataset_id_permanent
             # successfully, while the other instance will run into this
             # exception. Ignore this exception.
             pass
-        except google.api_core.exceptions.ResourceExhausted:
+        except Exception as exc:
+            # Don't fail the tests for unknown exceptions.
+            #
             # This can happen if we are hitting GCP limits, e.g.
             # google.api_core.exceptions.ResourceExhausted: 429 Quota exceeded
             # for quota metric 'Per project mutation requests' and limit
             # 'Per project mutation requests per minute per region' of service
             # 'cloudfunctions.googleapis.com' for consumer
             # 'project_number:1084210331973'.
             # [reason: "RATE_LIMIT_EXCEEDED" domain: "googleapis.com" ...
+            #
+            # It can also happen occasionally with
+            # google.api_core.exceptions.ServiceUnavailable when there is some
+            # backend flakiness.
+            #
             # Let's stop further clean up and leave it to later.
+            warnings.warn(f"Cloud functions cleanup failed: {str(exc)}")
             break