[Nginx] set event.module and event.dataset

[mtojek]

What does this PR do?

This PR adds event.module and event.dataset to the integration.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

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Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-06-29T22:28:25.941+0000

• Duration: 12 min 13 sec

• Commit: 6ff1d76

Test stats 🧪

Test	Results
Failed	0
Passed	40
Skipped	0
Total	40

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[elasticmachine]

Package system - 1.24.0 containing this change is available at https://epr.elastic.co/search?package=system

[elasticmachine]

Package cloud_security_posture - 1.2.13 containing this change is available at https://epr.elastic.co/search?package=cloud_security_posture

[elasticmachine]

Package cloud_security_posture - 1.5.2-preview1 containing this change is available at https://epr.elastic.co/search?package=cloud_security_posture

[elasticmachine]

Package panw - 3.20.0 containing this change is available at https://epr.elastic.co/search?package=panw