Override appengine_v1::DomainMappingsClient Retry Policies

This shows how to override the retry policies for appengine_v1::DomainMappingsClient:

  auto options =
      google::cloud::Options{}
          .set<google::cloud::appengine_v1::
                   DomainMappingsConnectionIdempotencyPolicyOption>(
              CustomIdempotencyPolicy().clone())
          .set<google::cloud::appengine_v1::DomainMappingsRetryPolicyOption>(
              google::cloud::appengine_v1::
                  DomainMappingsLimitedErrorCountRetryPolicy(3)
                      .clone())
          .set<google::cloud::appengine_v1::DomainMappingsBackoffPolicyOption>(
              google::cloud::ExponentialBackoffPolicy(
                  /*initial_delay=*/std::chrono::milliseconds(200),
                  /*maximum_delay=*/std::chrono::seconds(45),
                  /*scaling=*/2.0)
                  .clone());
  auto connection =
      google::cloud::appengine_v1::MakeDomainMappingsConnection(options);

  // c1 and c2 share the same retry policies
  auto c1 = google::cloud::appengine_v1::DomainMappingsClient(connection);
  auto c2 = google::cloud::appengine_v1::DomainMappingsClient(connection);

  // You can override any of the policies in a new client. This new client
  // will share the policies from c1 (or c2) *except* for the retry policy.
  auto c3 = google::cloud::appengine_v1::DomainMappingsClient(
      connection,
      google::cloud::Options{}
          .set<google::cloud::appengine_v1::DomainMappingsRetryPolicyOption>(
              google::cloud::appengine_v1::DomainMappingsLimitedTimeRetryPolicy(
                  std::chrono::minutes(5))
                  .clone()));

  // You can also override the policies in a single call:
  // c3.SomeRpc(..., google::cloud::Options{}
  //     .set<google::cloud::appengine_v1::DomainMappingsRetryPolicyOption>(
  //       google::cloud::appengine_v1::DomainMappingsLimitedErrorCountRetryPolicy(10).clone()));

Assuming you have created a custom idempotency policy. Such as:

class CustomIdempotencyPolicy : public google::cloud::appengine_v1::
                                    DomainMappingsConnectionIdempotencyPolicy {
 public:
  ~CustomIdempotencyPolicy() override = default;
  std::unique_ptr<
      google::cloud::appengine_v1::DomainMappingsConnectionIdempotencyPolicy>
  clone() const override {
    return std::make_unique<CustomIdempotencyPolicy>(*this);
  }
  // Override inherited functions to define as needed.
};