Add 'ReceivedMessage.delivery_attempt' field (via synth). (#9098)

pubsub/google/cloud/pubsub_v1/proto/pubsub.proto

@@ -737,6 +737,24 @@ message ReceivedMessage {
 
   // The message.
   PubsubMessage message = 2;
+
+  // Delivery attempt counter is 1 + (the sum of number of NACKs and number of
+  // ack_deadline exceeds) for this message.
+  //
+  // A NACK is any call to ModifyAckDeadline with a 0 deadline. An ack_deadline
+  // exceeds event is whenever a message is not acknowledged within
+  // ack_deadline. Note that ack_deadline is initially
+  // Subscription.ackDeadlineSeconds, but may get extended automatically by
+  // the client library.
+  //
+  // The first delivery of a given message will have this value as 1. The value
+  // is calculated at best effort and is approximate.
+  //
+  // If a DeadLetterPolicy is not set on the subscription, this will be 0.
+  // <b>EXPERIMENTAL:</b> This feature is part of a closed alpha release. This
+  // API might be changed in backward-incompatible ways and is not recommended
+  // for production use. It is not subject to any SLA or deprecation policy.
+  int32 delivery_attempt = 3;
 }
 
 // Request for the GetSubscription method.