Cherry-pick #26439 to 7.x: Enable configuring monitoring namespace

[michalpristas]

Cherry-pick of PR #26439 to 7.13 branch. Original message:

What does this PR do?

This PR adds a config option which is used as monitoring namespace.
This namespace is injected to data_stream section of the event and alters index name as well following our naming convention.

Config option is also reloadable and will be applied on change without a need of restarting agent

Why is it important?

Fixes: #25350

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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• Build Cause: Pull request #26555 opened

• Start Time: 2021-06-29T08:19:08.851+0000

• Duration: 86 min 5 sec

• Commit: ce14415

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Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948

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Test stats 🧪

Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948