Unexport all of Tracer's fields

[axw]

All options should be set at construction time, and should be immutable there on.

This used to be safe when the Tracer did not actively make requests to APM Server until events were enqueued, but this is no longer the case.

[apmmachine]

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• Start Time: 2022-02-24T05:30:22.249+0000

• Duration: 42 min 7 sec

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Test	Results
Failed	0
Passed	8328
Skipped	201
Total	8529

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[marclop]

Looks good!