Cherry-pick #20766 to 7.x: Request prometheus endpoints to be gzipped by default

[exekias]

Cherry-pick of PR #20766 to 7.x branch. Original message:

• Enhancement

What does this PR do?

This PR ensures that we add gzip content type to all prometheus endpoint poll requests.

Why is it important?

This improves performance network performance as most of the payload of exposition format compresses really well.

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.

Author's Checklist

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20795 opened]

• Start Time: 2020-08-26T09:30:19.946+0000

• Duration: 72 min 23 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3678
Skipped	810
Total	4488