[8.x](backport #5999) Add failureThreshold to elastic-agent self-monitoring config

[mergify[bot]]

What does this PR do?

Use failure_threshold introduced in https://github.com/elastic/beats/pull/41570 in self-monitoring configuration to avoid elastic-agent reporting DEGRADED if it fails to fetch metrics due to a component starting/stopping. The default value for the failure threshold is set to 2 but it can be configured via config file or fleet policy.

Why is it important?

It is important to avoid a misrepresentation of agent status due to a single metrics fetch erroring out once. See https://github.com/elastic/elastic-agent/issues/5332

Checklist

• [x] My code follows the style guidelines of this project
• [x] 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
• [x] 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

How to test this PR locally

Related issues

• Requires https://github.com/elastic/beats/pull/41570
• Closes #5332

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #5999 done by Mergify.

[elasticmachine]

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

[elastic-sonarqube[bot]]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 40%)

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