[Test failure] Monitoring samples test flake

```
1) Error:
metrics::monitoring::delete_metric_descriptor#test_0001_deletes a metric descriptor:
Google::Cloud::NotFoundError: 5:The metric '[custom.googleapis.com/my_metric_b18tl9g5kn](https://www.google.com/url?q=http://custom.googleapis.com/my_metric_b18tl9g5kn&sa=D)' does not exist.. debug_error_string:{UNKNOWN:Error received from peer ipv4:74.125.142.95:443 {grpc_message:"The metric \'[custom.googleapis.com/my_metric_b18tl9g5kn\](https://www.google.com/url?q=http://custom.googleapis.com/my_metric_b18tl9g5kn%5C&sa=D)' does not exist.", grpc_status:5, created_time:"2023-01-01T10:39:01.991609692+00:00"}}
/[h/.gem/gems/google-cloud-monitoring-v3-0.9.0/lib/google/cloud/monitoring/v3/metric_service/client.rb:764](https://cs.corp.google.com/piper///depot/google3/h/.gem/gems/google-cloud-monitoring-v3-0.9.0/lib/google/cloud/monitoring/v3/metric_service/client.rb?l=764):in `rescue in delete_metric_descriptor'
/[h/.gem/gems/google-cloud-monitoring-v3-0.9.0/lib/google/cloud/monitoring/v3/metric_service/client.rb:726](https://cs.corp.google.com/piper///depot/google3/h/.gem/gems/google-cloud-monitoring-v3-0.9.0/lib/google/cloud/monitoring/v3/metric_service/client.rb?l=726):in `delete_metric_descriptor'
/[workspace/google-cloud-monitoring/samples/metrics.rb:54](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/metrics.rb?l=54):in `delete_metric_descriptor'
/[workspace/google-cloud-monitoring/samples/acceptance/metrics_test.rb:47](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/acceptance/metrics_test.rb?l=47):in `block (5 levels) in '
/[workspace/google-cloud-monitoring/samples/acceptance/helper.rb:37](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/acceptance/helper.rb?l=37):in `block in retry_block'
/[workspace/google-cloud-monitoring/samples/acceptance/helper.rb:35](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/acceptance/helper.rb?l=35):in `loop'
/[workspace/google-cloud-monitoring/samples/acceptance/helper.rb:35](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/acceptance/helper.rb?l=35):in `retry_block'
/[workspace/google-cloud-monitoring/samples/acceptance/metrics_test.rb:46](https://cs.corp.google.com/piper///depot/google3/workspace/google-cloud-monitoring/samples/acceptance/metrics_test.rb?l=46):in `block (4 levels) in '
```

This might be a case of one test run's cleanup stomping on another test run.