Added MaxOpenRequests to kafka output config

[flexitrev]

Enhancement

Proposed commit message

Added MaxOpenRequests with a default of 5 to configuration options. This will increase throughput, but could have an impact on event ordering.

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• 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.

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[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @flexitrev? 🙏.
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-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[strawgate]

we dont expose idempotent as a setting and the default value for this also impacts message ordering so I think we can get rid of the comment about idempotent and message ordering?

[flexitrev]

Removed reference to idempotent, but I think it's worth keeping the note about the impact on message ordering. If we aren't explicit about the lack of guarantee, it will be easy for someone to get the wrong idea

[AndersonQ]

Don't it apply to all beats? Here it's listed as only affecting filebeat