Use WaitGroup to ensure go routine finishes sending to APM server

[estolfo]

Ensure that the go routine finishes sending to the APM server before the extension considers its work done and polls for the next event.

[ghost]

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• Reason: null

• Start Time: 2022-02-01T11:14:57.822+0000

• Duration: 12 min 17 sec

• Commit: d053807

Test stats 🧪

Test	Results
Failed	0
Passed	84
Skipped	0
Total	84

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[jlvoiseux]

Also passes the end-to-end tests developed on branch jlvoiseux/e2e-testing