Spanner: Ensure client backoff is implemented to respect client configuration. #5545
Description
There was an issue in the client library for python where requests to retry could occur without a proper retry object. In this case, rather than defaulting to an exponential backoff, we defaulted to immediate retry. Ensure that for this client library we respect the min, max, and multiplier as specified in the gapic configuration.
Original Bug: googleapis/google-cloud-python#7303