BigtableSession leak at #2658 is not fixed yet

[baeminbo]

The BigtableSession leak issue #2658 is not fixed yet because #2782 was reverted #2873.

This issue causes Dataflow pipeline with CloudBigtableIO to log error messages.

"*~*~*~ Channel ManagedChannelImpl{logId=43, target=bigtableadmin.googleapis.com:443} was not shutdown properly!!! ~*~*~*
    Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true." 

[igorbernstein2]

Hi, thanks for pointing this out. I re-opened the original issue.