Add support for ephemeral containers in elastic agent dynamic provider

[MichaelKatsoulis]

What does this PR do?

This PR adds support for ephemeral containers in elastic agent dynamic kubernetes provider.

Why is it important?

Ephemeral containers that are part of a pod can be monitored and be part of the published event.

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. Create a kind cluster with "feature-gates": "EphemeralContainers=true". example kind config (
   kind-ephemeral.yaml.zip)
2. Create en example pod

cat <<EOF | kubectl create -f - 
apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  containers:
  - image: mhausenblas/simpleservice:0.5.0
    name: app
EOF

3. Attach an ephemeral container to it kubectl debug -it example-pod --image=busybox --target=app
4. Run agent with pod resource enabled

providers.kubernetes:
  scope: cluster
  cleanup_timeout: 360s
  resources:
    pod:
       enabled: true

5. Run inspect command to check what is the compiled configuration (use proper path for the elastic-agent.yml config file :
   elastic-agent -c /etc/agent.yml -e inspect output -o default
6. Verify in the output that mapping is produced for the ephemeral container

processors:
    - add_fields:
        fields:
          container:
            id: a8d7f67674c848c27e71b7a97f3f82d9492080308a72153458a15df7485904bf
            image: busybox
            name: debugger-pbjkz
            runtime: containerd
          namespace: default
          pod:
            ip: 10.244.0.5
            labels: {}
            name: example-pod
            uid: d7a97d03-3baa-457c-b9e2-a8e80b831c23
        target: kubernetes

Related issues

• Closes [Agent] Support ephemeral containers in k8s provider  #27020

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Start Time: 2021-09-07T09:50:59.329+0000

• Duration: 82 min 49 sec

• Commit: 094f149

Test stats 🧪

Test	Results
Failed	0
Passed	7036
Skipped	16
Total	7052

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Test stats 🧪

Test	Results
Failed	0
Passed	7036
Skipped	16
Total	7052

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b support-ephemeral-container-k8s-provider upstream/support-ephemeral-container-k8s-provider
git merge upstream/master
git push upstream support-ephemeral-container-k8s-provider

[ChrsMark]

lgtm! We will also cover this with e2e in elastic/e2e-testing#1090