Store structured attributes from MongoDB logs in JSON format

[jsoriano]

What does this PR do?

Store the structured attributes included in JSON logs, using the flattened type for that.

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).

Related issues

• JSON logs support added in Add support for json logs in mongodb integration #1138.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #1153 updated

• Start Time: 2021-06-22T07:50:34.842+0000

• Duration: 18 min 47 sec

• Commit: 447aa0f

Test stats 🧪

Test	Results
Failed	0
Passed	29
Skipped	0
Total	29

Trends 🧪