Cherry-pick #22550 to 7.x: Trim down beat name if it's longer than 15 characters

[fearful-symmetry]

Cherry-pick of PR #22550 to 7.x branch. Original message:

What does this PR do?

This is a fix for #21297

If a beat name is longer than 15 characters on *nix systems, it won't match the process name found by the libbeat metrics tools, which will cause the beat to bomb out. If the beat name exceeds the name in /proc/self/comm, then truncate it.

Why is it important?

Custom beats with long names can fail.

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

How to test this PR locally

• create a beat with a name longer than 15 characters
• build, make sure it runs.

Related issues

• Beat names longer than 16 characters break the process stats reporting #21297

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Build stats

• Build Cause: [Pull request #22633 opened]

• Start Time: 2020-11-17T17:32:38.877+0000

• Duration: 71 min 7 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16388
Skipped	1368
Total	17756

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	16388
Skipped	1368
Total	17756