[Packetbeat] Fix Packetbeat parsing mongodb OP_MSG

[aleksmaus]

Proposed commit message

Fix Packetbeat parsing mongodb OP_MSG

• Fixes handling of OP_MSG based request/response, fixes missing "end" timestamp and "duration" field for the event.
• Cleanup

Addresses the issue #28858 that makes packetbeat unusable for any newer thatn 5.0 version of MongoDB.
The legacy "mongodb" property is not populated and probably we should not try to map the new protocol into the legacy data structures. Instead using the document from the request BSON ecoded body to populate the "query" and "request" (if requested) fields in the format compatible with the previous implmentation for now.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [Packetbeat]Packetbeat parsing mongodb OP_MSG #28858

Screenshots

Couple of examples of OP_MSG documents with query and request fields

[elasticmachine]

Pinging @elastic/sec-deployment-and-devices (Team:Security-Deployment and Devices)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @aleksmaus? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[fearful-symmetry]

I don't have a lot of MongoDB context, but the code looks fine.