Python DirectRunner _PubSubReadEvaluator creates new client per bundle

Lots of credential fetches.
Similar to https://issues.apache.org/jira/browse/BEAM-2264
but in this case the DirectRunner implementation seems to be creating a new client for each bundle:
https://github.com/apache/beam/blob/d5d7a7b7d0408d8435031e7bfce1abe2227115f5/sdks/python/apache_beam/runners/direct/transform_evaluator.py#L474

From: https://stackoverflow.com/questions/57010426/dataflow-access-to-pubsub-access-tokens