Override batch_v1::BatchServiceClient Retry Policies

 auto options =
 google::cloud::Options{}
 .set<google::cloud::batch_v1::
 BatchServiceConnectionIdempotencyPolicyOption>(
 CustomIdempotencyPolicy().clone())
 .set<google::cloud::batch_v1::BatchServiceRetryPolicyOption>(
 google::cloud::batch_v1::BatchServiceLimitedErrorCountRetryPolicy(
 3)
 .clone())
 .set<google::cloud::batch_v1::BatchServiceBackoffPolicyOption>(
 google::cloud::ExponentialBackoffPolicy(
 /*initial_delay=*/std::chrono::milliseconds(200),
 /*maximum_delay=*/std::chrono::seconds(45),
 /*scaling=*/2.0)
 .clone());
 auto connection =
 google::cloud::batch_v1::MakeBatchServiceConnection(options);
 // c1 and c2 share the same retry policies
 auto c1 = google::cloud::batch_v1::BatchServiceClient(connection);
 auto c2 = google::cloud::batch_v1::BatchServiceClient(connection);
 // You can override any of the policies in a new client. This new client
 // will share the policies from c1 (or c2) *except* from the retry policy.
 auto c3 = google::cloud::batch_v1::BatchServiceClient(
 connection,
 google::cloud::Options{}
 .set<google::cloud::batch_v1::BatchServiceRetryPolicyOption>(
 google::cloud::batch_v1::BatchServiceLimitedTimeRetryPolicy(
 std::chrono::minutes(5))
 .clone()));
 // You can also override the policies in a single call:
 // c3.SomeRpc(..., google::cloud::Options{}
 // .set<google::cloud::batch_v1::BatchServiceRetryPolicyOption>(
 // google::cloud::batch_v1::BatchServiceLimitedErrorCountRetryPolicy(10).clone()));

class CustomIdempotencyPolicy
 : public google::cloud::batch_v1::BatchServiceConnectionIdempotencyPolicy {
 public:
 ~CustomIdempotencyPolicy() override = default;
 std::unique_ptr<
 google::cloud::batch_v1::BatchServiceConnectionIdempotencyPolicy>
 clone() const override {
 return std::make_unique<CustomIdempotencyPolicy>(*this);
 }
 // Override inherited functions to define as needed.
};