Fix event.dataset in structlog

[basepi]

What does this pull request do?

In the logging integration, event.dataset should be a nested object, not a single key. This was causing issues noted in #1351

Related issues

Fixes #1351

[ghost]

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Build stats

• Start Time: 2021-10-13T15:11:11.583+0000

• Duration: 28 min 43 sec

• Commit: a6cecf4

Test stats 🧪

Test	Results
Failed	0
Passed	10007
Skipped	8977
Total	18984

💚 Flaky test report

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