title: Google PubSub - Ack a message using ack_id
body: <p>I have an architecture made of:</p>
  <ul>
  <li>PubSub Topic 'A'</li>
  <li>Subscription 'B' on topic 'A' that pushes messages to the endpoint 'X'</li>
  <li>Cloud Function 'C' triggered by the endpoint 'X', runtime in Python</li>
  </ul>
  <p>Everytime a new message is published on the topic 'A', the subscription 'B' pushes it to the endpoint 'X' that triggers the Cloud Function 'C'.</p>
  <p>The problem I'm facing is the 'acking' of the message.
  I'd add a step to the Cloud Function 'C' in order to manually ack the message with the ack_id but every thread I've found discourages the manual acking of the messages using the ack_id and I don't know how to do in a different way...</p>
  <p>Any suggestion?
  How can I manually ack messages?</p>
  <p>Thanks in advance!</p>
  <p>ps. This architecture is necessary because topic 'A' and Cloud Function 'C' stay in different projects</p>