[8.x](backport #5492) Add apm trace sampling

[mergify]

What does this PR do?

This PR adds the possibility to specify apm sample rate in elastic agent APM configuration

Why is it important?

Specifying a sample rate allows controlling the ingestion rate, reducing overhead and storage requirements when self-monitoring is active

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

• Closes Add missing tracing sampling APM config section #5211

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 #5492 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b284e56 has failed:

On branch mergify/bp/8.x/pr-5492
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit b284e56ccf.
  (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:   NOTICE.txt
	modified:   internal/pkg/agent/application/apm_config_modifier_test.go
	modified:   internal/pkg/agent/application/coordinator/diagnostics_test.go
	modified:   internal/pkg/agent/cmd/run.go
	modified:   internal/pkg/core/monitoring/config/config.go
	modified:   internal/pkg/core/monitoring/config/config_test.go
	modified:   pkg/component/runtime/apm_config_mapper.go
	modified:   pkg/component/runtime/command_test.go
	modified:   pkg/component/runtime/manager_fake_input_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   go.mod
	both modified:   go.sum

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

[elasticmachine]

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pchila? 🙏

[elastic-sonarqube]

Quality Gate failed

Failed conditions
20.0% Coverage on New Code (required ≥ 40%)

See analysis details on SonarQube