ARROW-15781: [Python] Release GIL in ensure_complete_metadata
ARROW-15781: [Python] Release GIL in ensure_complete_metadata

Closes #12501.

Closes #12509 from lidavidm/arrow-15781

Authored-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>
Signed-off-by: Joris Van den Bossche <36c737f86d71d8ee1ddeb1e5c83102ee5de8f97c@gmail.com>

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