A message and its corresponding acknowledgment ID.
.. attribute:: ack_id
This ID can be used to acknowledge the received message.
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
EXPERIMENTAL: 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.
