Cherry-pick #24622 to 7.x: accept text/plain type by default for prometheus client scraping

[ChrsMark]

Cherry-pick of PR #24622 to 7.x branch. Original message:

What does this PR do?

accept text/plain type by default for prometheus client scraping

Why is it important?

prometheus java client 0.10.0 introduced breaking changes that when application/openmetrics-text is seen in accept list, new types(i.e. info) will be exposed and they are not supported by current metrics parser

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

• [ ]

How to test this PR locally

• start a java app instrumented with prometheus client version 0.10.0 and then start metricbeat to scrape it

Related issues

• Closes new openmetrics types are not supported for prometheus module #24554

Use cases

Screenshots

Logs

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24731 opened

• Start Time: 2021-03-24T14:55:40.394+0000

• Duration: 100 min 16 sec

• Commit: 66e1baa

Test stats 🧪

Test	Results
Failed	0
Passed	8896
Skipped	2388
Total	11284

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	8896
Skipped	2388
Total	11284