Optional. The name of the topic to which dead letter messages should be
published. Format is `projects/{project}/topics/{topic}`.The Pub/Sub
service account associated with the enclosing subscription's parent project
(i.e., service-{project_number}@gcp-sa-pubsub.iam.gserviceaccount.com) must
have permission to Publish() to this topic.

The operation will fail if the topic does not exist.
Users should ensure that there is a subscription attached to this topic
since messages published to a topic with no subscriptions are lost.

string dead_letter_topic = 1 [(.google.api.field_behavior) = OPTIONAL];

Optional. The name of the topic to which dead letter messages should be
published. Format is `projects/{project}/topics/{topic}`.The Pub/Sub
service account associated with the enclosing subscription's parent project
(i.e., service-{project_number}@gcp-sa-pubsub.iam.gserviceaccount.com) must
have permission to Publish() to this topic.

The operation will fail if the topic does not exist.
Users should ensure that there is a subscription attached to this topic
since messages published to a topic with no subscriptions are lost.

string dead_letter_topic = 1 [(.google.api.field_behavior) = OPTIONAL];

Optional. The maximum number of delivery attempts for any message. The
value must be between 5 and 100.

The number of delivery attempts is defined as 1 + (the sum of number of
NACKs and number of times the acknowledgement deadline has been exceeded
for the message).

A NACK is any call to ModifyAckDeadline with a 0 deadline. Note that
client libraries may automatically extend ack_deadlines.

This field will be honored on a best effort basis.

If this parameter is 0, a default value of 5 is used.

int32 max_delivery_attempts = 2 [(.google.api.field_behavior) = OPTIONAL];