Enrich k8s state service events & add selector meta

[ChrsMark]

What does this PR do?

1. adds metadata enricher on state_service metrics so as to enrich metrics with Service objects metadata.
2. make use of ServiceMetadataGenerator to enrich Service related events and not the generic ResourceMetadataGenerator
3. Improves ServiceMetadataGenerator so as to include selector information (if exist) for a Service object.

Note that with this implementation selector's key:val maps are added similarly with labels like kubernetes.service.selectors.* but we can add them differently if Metrics UI requires that, @sorantis just let me know what you think.

Why is it important?

So as to properly enrich state_service events with Service metadata and include selector information in these metadata.

Example:

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376

This service will be enriched with kubernetes.service.selectors.app: MyApp which will give us to the option to correlate the service with all the pods that are selected/exposed by this service and are labeled with kubernetes.labels.app: MyApp.

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.

How to test this PR locally

1. Deploy Metricbeat on k8s to collect metrics from kube_state_metrics and specifically using state_service metricset.
2. Ensure that metadata are included (ie labels) as well as selector information.

Related issues

• Closes Enrich Kubernetes Service metadata with ServiceMetadataGenerator #22444
• Relates to Migrate Kubernetes integration integrations#511

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Pinging @elastic/integrations (Team:Integrations)

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Build stats

• Build Cause: Pull request #23730 event

• • Start Time: 2021-02-01T08:09:39.533+0000

• Duration: 50 min 33 sec

• Commit: 55fc231

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Test	Results
Failed	0
Passed	44578
Skipped	4612
Total	49190

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Test stats 🧪

Test	Results
Failed	0
Passed	44578
Skipped	4612
Total	49190

[jsoriano]

Nice 👍