Remove custom_sampling_context

[sentrivana]

• Add support for the sampled flag for start_span and respect it when making sampling decisions.
• Rework sampling_context in traces_sampler to work with span attributes instead. Make sure we still have the same data accessible as now.

We could go one step further and change the format of sampling_context to just be the actual span attributes without any postprocessing into the current format. I kept the format in line with what we have now to make it easier to update.

See #3746
Closes #3739

This is a breaking change since we're removing custom_sampling_context. It'll break multiple integrations until we fix them (see #3746).

[codecov]

❌ 1863 Tests Failed:

Tests completed	Failed	Passed	Skipped
19899	1863	18036	4412

View the top 3 failed tests by shortest run time

tests.test_scope test_with_isolation_scope_data

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     - }

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

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_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

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[sentrivana]

This is what this would effectively look like for an integration: https://github.com/getsentry/sentry-python/pull/3764/files