Add esclient stats

[gsoldevila]

What does this PR do?

This PR updates the Kibana module of metricbeat, enriching the stats metricset with connectivity information from elasticsearch-js client, exposed in /api/stats through #141434.

Why is it important?

The goal is for metricbeat to ingest these new fields, and ship them into the overview cluster, for internal consumption. This information might be very relevant for SDH investigations, whenever there are performance concerns or connectivity errors. We can contrast it with response times to find correlations, or even analyse it individually to detect anomalies (e.g. workload not being properly balanced across ES nodes).

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

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How to test this PR locally

Related issues

• Relates to #141434

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[mergify]

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If this is a bug or security fix, could you label this PR @gsoldevila? 🙏.
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• The upcoming major version of the Elastic Stack
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• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-11-18T11:21:47.977+0000

• Duration: 54 min 21 sec

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Test	Results
Failed	0
Passed	3931
Skipped	877
Total	4808

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[crespocarlos]

LGTM!

@klacabane, we probably need to add these new fields to Kibana integration package.

[rudolf]

@crespocarlos We're not able to merge this, will you do that?