Fix stat tests

[RLKRo]

Description

This PR fixes issues I found with stats tests while investigating workflow failures:

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This PR fixes an issue with randomly failing workflows:

test_get_current_label was using a pipeline from tutorials.stats.1_extractor_functions which had dff_instrumentor = OtelInstrumentor.from_url("grpc://localhost:4317") and unlike other tests it was not reinstrumented with an InMemoryExporter.

test_get_current_label is changed in the following way:

• A new pipeline is created for that test instead of importing it from a tutorial.
• Fix the way result is asserted (previously it did not fail when it should have).

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This PR updates the way instrumentor is reinstrumented in test_otlp_integration to the same way it is done in test_tutorials (by reinstrumenting an instrumentor from an imported tutorial instead of creating a new one).

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This PR replaces example with tutorial in stat tests.

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This PR removes unused expected parameter from test_otlp_integration.

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Also, while working on this issue I noticed that otel_logs have a null timestamp:

However, otel_traces does not have the same issue

Checklist

• I have covered the code with tests
• I have added comments to my code to help others understand it
• I have updated the documentation to reflect the changes
• I have performed a self-review of the changes