Address partial list of code review comments

[estolfo]

This PR addresses some comments from this code review: #124 (comment)

Todo (3 March)

• Use JSON.Decoder

[ghost]

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• Start Time: 2022-03-09T10:53:11.422+0000

• Duration: 7 min 38 sec

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[stuartnelson3]

looks good! made some comments but i think they're mostly cosmetic

[trentm]

FYI: When testing with a build of the extension using this branch and with the Node.js APM agent changes to send the ?flushed=true signal (elastic/apm-agent-nodejs#2586), I no longer get the panics I reported in #133

[estolfo]

Thanks for the update @trentm. This change should have indeed fixed that panic.
Also, @jlvoiseux has written a test in this PR that forces the scenario and confirms that the changes linked above resolve the panic.

[stuartnelson3]

looks good!