Cherry-pick #23730 to 7.x: Enrich k8s state service events & add selector meta

[ChrsMark]

Cherry-pick of PR #23730 to 7.x branch. Original message:

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

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[botelastic]

This pull request doesn't have a Team:<team> label.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #23806 updated

• • Start Time: 2021-02-02T14:20:25.737+0000

• Duration: 47 min 11 sec

• Commit: 2e66811

Test stats 🧪

Test	Results
Failed	0
Passed	44779
Skipped	4647
Total	49426

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	44779
Skipped	4647
Total	49426