Update agent monitoring config to support /inputs of filebeat

[kcreddy]

What does this PR do?

• Adds agent configuration to monitor filebeat's /inputs/ endpoint.
  Reference: [Metricbeat] beat module - add metricset for input metrics beats#33953 (comment)

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

Related issues

• Relates [Metricbeat] beat module - add metricset for input metrics beats#33953
• Relates https://github.com/elastic/security-team/issues/5525

[mergify]

This pull request does not have a backport label. Could you fix it @kcreddy? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Start Time: 2023-02-02T05:04:50.358+0000

• Duration: 17 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4889
Skipped	13
Total	4902

💚 Flaky test report

Tests succeeded.

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.333% (59/60)	👍
Files	69.082% (143/207)	👍
Classes	68.514% (272/397)	👍
Methods	53.772% (834/1551)	👍
Lines	39.046% (9093/23288)	👎 -0.099
Conditionals	100.0% (0/0)	💚

[kcreddy]

/test

[michalpristas]

lgtm now

[kcreddy]

/test