[exporter/datadog] Add missing package comments

[songy23]

Description:

Add missing package comments for datadogexporter sub-packages. Fix lint errors.

Link to tracking Issue:

Testing:

Documentation:

[songy23]

Thanks for the review! Please add label Skip Changelog

[runforesight]

Foresight Summary

    
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Foresight hasn't detected any major impact on your workflows and tests.

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 build-and-test workflow has finished in 36 minutes 15 seconds (23 minutes 4 seconds less than main branch avg.) and finished at 16th Dec, 2022.

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Job	Failed Steps	Tests
integration-tests	N/A	N/A	See Details

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