Flaky celery test with pypy-3.8 ubuntu

[xrmx]

=================================== FAILURES ===================================
________________________ TestMetrics.test_basic_metric _________________________

self = <tests.test_metrics.TestMetrics testMethod=test_basic_metric>

    def test_basic_metric(self):
        CeleryInstrumentor().instrument()
        start_time = default_timer()
        task_runtime_estimated = (default_timer() - start_time) * 1000
    
        metrics = self.get_metrics()
        CeleryInstrumentor().uninstrument()
>       self.assertEqual(len(metrics), 1)
E       AssertionError: 0 != 1

instrumentation/opentelemetry-instrumentation-celery/tests/test_metrics.py:46: AssertionError

CI run: https://github.com/open-telemetry/opentelemetry-python-contrib/actions/runs/15156691327/job/42613075177

[jomcgi]

This should be resolved when we drop support for Python 3.8
I've rebased #3399 so should be able to get final reviews / merge soon.

[rbagd]

Not sure if this specifically Python 3.8 related, albeit definitely more common. I've hit similar issues in other versions as well.

I've reworked Celery metrics instrumentation recently in this PR, and with that the tests seem more robust to flakiness, albeit one does have to wait for tasks to finish properly.