Add support for json logs in mongodb integration

[jsoriano]

What does this PR do?

Add support for MongoDB logs in json format.

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

• Syncs changes introduced in Beats in [Filebeat] [MongoDB] Support MongoDB 4.4 json logs beats#24774 and Store message from MongoDB json logs in message field beats#26338.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #1138 updated

• Start Time: 2021-06-17T09:40:13.381+0000

• Duration: 14 min 19 sec

• Commit: 796616e

Test stats 🧪

Test	Results
Failed	0
Passed	29
Skipped	0
Total	29

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