Log message payload in subscription success

[jBonoraW]

Log message payload in post_process_message_success.

Now we have the post_process_message_failure and post_publish_failure logging the gcloud Message, we are also changing that to only log the message payload.

The gcloud library uses this representation for the message and the data it's truncated in 50 characters:

_MESSAGE_REPR = """\
Message {{
  data: {!r}
  ordering_key: {!r}
  attributes: {}
}}"""