fix: workaround BigtableIO.read() recreating the client for every split #32724
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This is a temporary workaround for BigtableIO.read().
Currently BigtableIO.read() uses the BoundedSource api, which means that its reader is closed per split. Originally a ref count system was added to allow different reader instances to share the same bigtable client. Unfortunately that system proved to be inadequate - in practice the the readers from multiple threads get closed around the same time.
The proper fix for this would be to implement Bigtable.read() as a SplittableDoFn and tie in to the worker lifetime via the Teardown event. However that will be a longer term project. For the short term future, this patch will simply defer the decrementing the refcount by 10 secs, which should prevent the ref count from dropping to zero in between splits.
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