PubSub: use debug for logging the inserts

[benjben]

In collector 2.8.2 this line was logged with log level DEBUG.

In 2.9.0 the log level became INFO, to be consistent with Kinesis.

But for PubSub the events are not batched and they are inserted one by one, resulting in a lot of logs. The log level for this line should be put back to DEBUG.