Cherry-pick #25113 to 7.x: [Metricbeat] Remove elasticsearc.index.created from the SM code

[sayden]

Cherry-pick of PR #25113 to 7.x branch. Original message:

What does this PR do?

SM index metricset does an API HTTP call to elasticsearch to retrieve index metadata information. This information is required to populate field elasticsearch.index.created or (index_stats.created if xpack.enabled: true when configuring the module).

The code removes the API call to /_cluster/state/metadata.

Why is it important?

The HTTP call to /_cluster/state/metadata can be VERY expensive. The reason for this is that it returns the entire mapping of all indices on a cluster. If a user has many indices and / or each index has many fields, this could lead to very heavy responses from elasticsearchr (20 Mb in big clusters measured with curl).

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

• This is a breaking change, because users could be using field elasticsearch.index.created or index_stats.created and they will be removed.
• Stack Monitoring UI seems to keep working without any issues. I don't see this field populated anywhere.

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

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• Build Cause: Pull request #25172 opened

• Start Time: 2021-04-20T11:50:51.160+0000

• Duration: 78 min 13 sec

• Commit: 5aee3b5

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Failed	0
Passed	8357
Skipped	2270
Total	10627

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Passed	8357
Skipped	2270
Total	10627

[sayden]

Changelog has been included and the files touched are the expected ones. Merging