Expose output disk queue settings when running under agent

[pchila]

Proposed commit message

See title

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.

Disruptive User Impact

No disruption as the disk queue was already unsupported when running under agent

Author's Checklist

• [ ]

How to test this PR locally

Package agent using beats binaries that include this change.
Install agent as standalone and modify the output in elastic-agent.yml

outputs:
  default:
    type: elasticsearch
    hosts: [127.0.0.1:9200]
    api_key: "example-key"
    #username: "elastic"
    #password: "changeme"
## Note: we need to disable the preset otherwise it will override queue.disk with queue.mem setting
    #preset: balanced
    queue.disk:
        max_size: 1GB

Add a beats input (filebeat or other) that uses the output and the disk queue will appear under the input data directory, for example:

root@elastic-agent-dev:/opt/Elastic/Agent/data/elastic-agent-9.0.0-SNAPSHOT-d99b09/run/filestream-default/diskqueue# ll
total 8
drwxr-xr-x 2 root root 4096 Sep 17 09:18 ./
drwxrwx--- 4 root root 4096 Sep 17 09:18 ../
-rw------- 1 root root    0 Sep 17 09:18 state.dat

Related issues

Use cases

Screenshots

Logs

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @pchila? 🙏.
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.