implement dropping of fast exit spans

[beniwohli]

closes #1309

[beniwohli]

this change was needed so that the tests that use this fixture also respect the default settings we set up in the elasticapm_client fixture

[ghost]

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• Start Time: 2021-12-01T14:23:00.098+0000

• Duration: 34 min 47 sec

• Commit: 76a6cc1

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Test	Results
Failed	0
Passed	10269
Skipped	9579
Total	19848

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[basepi]

I had a hunch this was where the conversation about adding microseconds happened, so I checked the spec and added one note below. Otherwise looks great!