Fix and re-enable the flaky celery tests

Some celery tests were disabled a long time ago because of inconsistency.

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py#L36
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py#L146
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py#L193
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py#L209
https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py#L479

All of the above tests either use .delay() or .apply_async() to trigger the Celery tasks. This means a span is generated for the producer, then a message is put on a queue/broker. A celery worker then picks up the message and executes the relavant task which generates the consumer span. This is tricky as there can be some delay between producing and consuming. It looks like our tests do not take this delay into account and expect both producer and consumer spans to be available immediately after calling delay/apply_async which obviously fails a lot of times.

Solution here would be to wait till the expected number of spans arrive to memory exporter or wait on a condition fulfilled by the task to be executed. Whichever solution we pick should have a timeout with a assertion error so tests don't get stuck forever in case the message never arrives.

tests/opentelemetry-docker-tests/tests/celery/test_celery_functional.py から始め、参照されている行の無効化されたテストと、メモリエクスポーターがどのようにスパンを監視しているかを確認します。Celery の機能テストファイルを実行して、タイミングによる失敗を再現します。テストが再び有効化され、タイムアウト付きで producer と consumer のスパンを待機し、タスクが到着しない場合にハングするのではなく assertion によって失敗すれば完了です。