[9.2] (backport #11619) Add default option to allow merging multiple agent config keys

[mergify]

What does this PR do?

Merge multiple agent.* keys when handling config.

Why is it important?

Multiple keys were getting silently ignored, leading to configs that contain:

agent.logging.level: error
agent.logging.to_files: true

not working as expected.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

Standalone agent configs will no longer silently ignore multiple agent.* attributes.

How to test this PR locally

Create a stand-alone config with multiple agent.* entries such as:

agent.logging.level: debug
agent.logging.to_stderr: true
agent:
  download:
    sourceURI: https://artifacts.elastic.co/downloads/
  monitoring:
    enabled: true
    use_output: default
    logs: false
    metrics: false
    traces: true
    namespace: default

and observe that the logging entries are configured correctly.

Related issues

• Closes Configure logging for standalone Elastic Agents is not working #3717

  ---

  This is an automatic backport of pull request Add default option to allow merging multiple agent config keys #11619 done by Mergify.

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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• Commit: 6f367b0

cc @michel-laterman