Cherry-pick #19699 to 7.x: Fix k8s scheduler compatibility issue

[ChrsMark]

Cherry-pick of PR #19699 to 7.x branch. Original message:

What does this PR do?

This PR introduces OpSetSuffix in order to solve a compatibility issue in Kubernetes Scheduler metricset.
In older version of k8s the Prometheus metric scheduler_pod_preemption_victims was a Gauge but recently they changed it
to a Histogram. Until now we have been mapping this Gauge metric to scheduling.pod.preemption.victims.count field. This makes it impossible for the module to parse Prometheus metrics from newer versions of Scheduler where this filed is a Histogram. With this PR OpSetSuffix can try to check if the value of this metric is numeric and extend it with counter suffix otherwise it is of Histogram type and we do nothing. This makes scheduling.pod.preemption.victims.count filed to be actually numeric in all cases since Histograms create this filed by default and when it is a Gauge we leverage the OpSetSuffix to do the mapping appropriately.

Why is it important?

In order to preserve compatibility with old and new versions of Prometheus metrics exported by Kubernetes Scheduler.

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.

Related issues

• Closes k8s scheduler broken mapping #19332

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #19723 opened]

• Start Time: 2020-07-08T07:09:44.041+0000

• Duration: 77 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2972
Skipped	685
Total	3657

[kaiyan-sheng]

looks good!