Implement a retry policy decorator to handle Internal errors in tests

In #5851, we added manual retry logic inside `try_get_metric` to catch and ignore transient `Code::Internal` and `Code::NotFound` errors from the Cloud Monitoring backend.

While this works for now to reduce test flakiness, we should implement a proper retry policy decorator that can automatically wrap these API calls and handle transient errors (like `Internal`) under the hood.

See https://github.com/googleapis/google-cloud-rust/pull/5851#pullrequestreview-4439784691