fix issue in span serialization when using APM Server < 7.16

[beniwohli]

the issue wasn't originally caught in testing because we hard code
the server_version to a certain value (currently 8.0.0), and this
bug was in a branch that only is hit with a server version lower
than 7.16

fixes #1509

[ghost]

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• Start Time: 2022-03-30T13:58:08.171+0000

• Duration: 20 min 31 sec

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Skipped	3208
Total	8028

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