[v3-1-test] Flush in-memory OTel metrics at process shutdown (#61808)

[xBis7]

Manual backport for #61808

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[potiuk]

Rebased - but there were some strange errors?

[xBis7]

It's the new unit test that I added.

https://github.com/apache/airflow/pull/61869/changes#diff-62c84d713d57cf60b8db5b418f5c37f2490555fddf66e0b88e8f6542f1c7e076R320-R321

test_module_name = "unit.core.test_otel_logger"
function_call_str = f"import {test_module_name} as m; m.mock_service_run()"

When it tries to execute the import, we get an error that the module unit.core isn't found.

[xBis7]

@potiuk I fixed it and also did another rebase. The CI is green. Can you take another look?

[potiuk]

Cool! Thanks!