I believe this change can reduce the gRPC connection leak error messages that look scary but no impact on the Bigtable operations.

*~*~*~ Channel ManagedChannelImpl{logId=288, target=bigtableadmin.googleapis.com:443} was not shutdown properly!!! ~*~*~* Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true.

Reader.close(): fixes #2658 where original fix #2782 was rolled back by #2871 and #2873. As we don’t use “session” here anymore, I believe we should close “connection” here.

AbstractCloudBigtableTableDoFn.tearDown(): If an exception happened in ProcessElement, FinishBundle is not executed. We should clean up resources in TearDown. You can see the similar code with BigQueryIO apache/beam#14949.

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Fixes #2658

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