Many subscribers created and sometimes cleanup issues

Hi. We're using this connector and generally it works well but from time to time I see SEVERE errors in the log like below. When looking into this issue I realize that the connector rather frequently (around 5-10 per 15 min interval) reports creating a new subscriber. I wonder if this is normal, and of course in particular I'm interested to know if some additional cleanup may be needed to avoid the SEVERE messages.

```[2019-09-26 10:38:32,304] INFO WorkerSourceTask{id=CPSElwatchConnector-0} Finished commitOffsets successfully in 13 ms (org.apache.kafka.connect.runtime.WorkerSourceTask:496)
[2019-09-26 10:39:17,244] INFO Creating subscriber. (com.google.pubsub.kafka.source.CloudPubSubGRPCSubscriber:66)
Sep 26, 2019 10:39:17 AM io.grpc.internal.ManagedChannelImpl$ManagedChannelReference cleanQueue
SEVERE: *~*~*~ Channel io.grpc.internal.ManagedChannelImpl-91 for target pubsub.googleapis.com:443 was not shutdown properly!!! ~*~*~*
Make sure to call shutdown()/shutdownNow() and awaitTermination().```