Unpack custom_sampling_context into attributes in ASGI

[sentrivana]

Replace custom_sampling_context={"asgi_scope": scope} with several span attributes. Also, add sampling_context["transaction_context"]["source"], which was previously missing.

With the switch to OTel, custom_sampling_context can't continue to work as before. We've opted for making all necessary data accessible in the traces_sampler via span attributes instead. In order to use span attributes, we need to unpack any objects with serializable OTel-friendly data.

Part of #3746

[codecov]

❌ 1613 Tests Failed:

Tests completed	Failed	Passed	Skipped
19831	1613	18218	4413

View the top 1 failed tests by shortest run time

tests.test_utils test_datetime_from_isoformat[2021-01-01T00:00:00.000000-00:00-expected_output4]

Stack Traces | 0.001s run time

tests/test_utils.py:109: in test_datetime_from_isoformat
    assert datetime_from_isoformat(input_str) == expected_output, input_str
E   NameError: name 'datetime_from_isoformat' is not defined

View the full list of 2 ❄️ flaky tests

tests.test_utils test_datetime_from_isoformat[2021-01-01T00:00:00.000000-0000-expected_output6]

Flake rate in main: 50.00% (Passed 32 times, Failed 32 times)

Stack Traces | 0.001s run time

tests/test_utils.py:109: in test_datetime_from_isoformat
    assert datetime_from_isoformat(input_str) == expected_output, input_str
E   NameError: name 'datetime_from_isoformat' is not defined

tests.test_scope test_with_isolation_scope_data

Flake rate in main: 99.11% (Passed 2 times, Failed 223 times)

Stack Traces | 0.001s run time

tests/test_scope.py:387: in test_with_isolation_scope_data
    assert scope._tags == {"before_isolation_scope": 1}
E   AssertionError: assert {} == {'before_isolation_scope': 1}
E     
E     Right contains 1 more item:
E     {'before_isolation_scope': 1}
E     
E     Full diff:
E     + {}
E     - {
E     -     'before_isolation_scope': 1,
E     - }

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[sentrivana]

Making this a draft, will try to fix sampler-related asgi tests here too