Override spanner_admin::InstanceAdminClient Retry Policies

  namespace spanner = ::google::cloud::spanner;
  [](std::string const& project_id) {
    // An instance admin client is controlled by three policies. The retry
    // policy determines for how long the client will retry transient failures.
    auto retry_policy =
        google::cloud::spanner_admin::InstanceAdminLimitedTimeRetryPolicy(
            /*maximum_duration=*/std::chrono::minutes(25))
            .clone();
    // The backoff policy controls how long does the client waits to retry after
    // a transient failure. Here we configure a truncated exponential backoff
    // with jitter:
    //   https://en.wikipedia.org/wiki/Exponential_backoff
    //   https://cloud.google.com/storage/docs/exponential-backoff
    auto backoff_policy = spanner::ExponentialBackoffPolicy(
                              /*initial_delay=*/std::chrono::seconds(2),
                              /*maximum_delay=*/std::chrono::minutes(10),
                              /*scaling=*/2.0)
                              .clone();
    // The polling policy controls how the client waits for long-running
    // operations (such as creating new instances). `GenericPollingPolicy<>`
    // combines existing policies.  In this case, keep polling until the
    // operation completes (with success or error) or 45 minutes, whichever
    // happens first. Initially pause for 10 seconds between polling requests,
    // increasing the pause by a factor of 4 until it becomes 2 minutes.
    auto polling_policy =
        spanner::GenericPollingPolicy<>(
            spanner::LimitedTimeRetryPolicy(
                /*maximum_duration=*/std::chrono::minutes(45)),
            spanner::ExponentialBackoffPolicy(
                /*initial_delay=*/std::chrono::seconds(10),
                /*maximum_delay=*/std::chrono::minutes(2),
                /*scaling=*/4.0))
            .clone();
    auto client = google::cloud::spanner_admin::InstanceAdminClient(
        google::cloud::spanner_admin::MakeInstanceAdminConnection(),
        google::cloud::Options{}
            .set<google::cloud::spanner_admin::InstanceAdminRetryPolicyOption>(
                std::move(retry_policy))
            .set<
                google::cloud::spanner_admin::InstanceAdminBackoffPolicyOption>(
                std::move(backoff_policy))
            .set<
                google::cloud::spanner_admin::InstanceAdminPollingPolicyOption>(
                std::move(polling_policy)));

    // Use the client as usual.
    std::cout << "Available configs for project " << project_id << "\n";
    auto project = google::cloud::Project(project_id);
    for (auto& cfg : client.ListInstanceConfigs(project.FullName())) {
      if (!cfg) throw std::move(cfg).status();
      std::cout << cfg->name() << "\n";
    }
    std::cout << "End of available configs\n";
  }

class CustomIdempotencyPolicy : public google::cloud::spanner_admin::
                                    InstanceAdminConnectionIdempotencyPolicy {
 public:
  ~CustomIdempotencyPolicy() override = default;
  std::unique_ptr<
      google::cloud::spanner_admin::InstanceAdminConnectionIdempotencyPolicy>
  clone() const override {
    return std::make_unique<CustomIdempotencyPolicy>(*this);
  }
  // Override inherited functions to define as needed.
};