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

[mergify]

What does this PR do?

Use failure_threshold introduced in elastic/beats#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 #5332

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/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 Metricbeat: add configurable failure threshold before reporting streams as degraded beats#41570
• Closes Agent gets unhealthy temporarily because Beat monitoring sockets are not available #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?
• ...

---

This is an automatic backport of pull request #5999 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2a46509 has failed:

On branch mergify/bp/8.15/pr-5999
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit 2a4650974e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   internal/pkg/agent/application/coordinator/diagnostics_test.go
	modified:   internal/pkg/agent/application/monitoring/v1_monitor_test.go
	modified:   internal/pkg/core/monitoring/config/config.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/agent/application/monitoring/v1_monitor.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[pchila]

Closing this backport as elastic/beats#40565 is not on 8.15 branch