[9.3] (backport #11998) (bugfix) log level does not change when standalone agent is reloaded or when otel runtime is used

[mergify]

What does this PR do?

When agent.logging.level config is changed during runtime in standalone elastic-agent, the new log level was not respected. (Note: This works in managed mode. This is the code path followed by managed mode for ref https://github.com/khushijain21/elastic-agent/blob/loglevel/internal/pkg/agent/application/actions/handlers/handler_action_settings.go#L54-L94 )

This PR fixes this bug and ensures this works both for process and OTel runtime.

Note: If both agent.logging.level and service::telemetry::logs::level are set - then the agent log level is applied to process runtime and telemetry::logs is applied to OTel runtime. If not, all components use agent.logging.level

Why is it important?

This is important so that we don't have to restart elastic-agent to update its logging level

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

None

How to test this PR locally

Start standalone elastic-agent with monitoring enabled and change the logging level without restarting. Observe the debug logs.

I tested this PR manually for both process and otel mode. And by switching from INFO-DEBUG-INFO.

Related issues

• Closes Log level changes do not affect the elastic-otel-collector sub-process #11959

---

This is an automatic backport of pull request #11998 done by [Mergify](https://mergify.com).

[elasticmachine]

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

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: ce03707

Failed CI Steps

• x86_64:sudo: fleet-endpoint-security

cc @khushijain21