Fix counter metrics for ParDo#with_exception_handling(timeout).

[claudevdm]

Make parent thread state sampler available in child thread when running ParDo#with_exception_handling(timeout).

• ParDo#with_exception_handling(timeout) executes the DoFn in a new thread [1]
• Incrimenting a counter invokes tries to access the state sampler that was declared in the main thread local storage from the child thread [2]
• Since the child thread does not have access to the state sampler from the main thread the get_current_tracker method fails silently [3] and the counter isnt actually incremented

This change makes the parent thread's state sampler available to the child thread.

[1]

beam/sdks/python/apache_beam/transforms/core.py

Line 2599 in 1eddbdc

return self._pool.submit(

[2]

beam/sdks/python/apache_beam/runners/worker/statesampler.py

Line 43 in 1eddbdc

_STATE_SAMPLERS = threading.local()

[3]

beam/sdks/python/apache_beam/runners/worker/statesampler.py

Line 54 in 1eddbdc

return None

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