Improve shutdown logs

[AndersonQ]

What does this PR do?

Improve logs when handling shutdown.

Why is it important?

Often we investigate problems which one of the symptom is the Elastic Agent or beats restarting. However often it's uncertain what is making the agent to shutdown.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Related issues

• Relates Add logs that give more information about why agent is shutting down elastic-agent#1358

[elasticmachine]

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• Start Time: 2022-10-26T20:55:14.371+0000

• Duration: 10 min 2 sec

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Test	Results
Failed	0
Passed	723
Skipped	5
Total	728

🤖 GitHub comments

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[faec]

shutdowns -> "shuts down"