feat(kubelet): Create an agent container in the kubelet pod only

[tormodmacleod]

Description

This provides the user with the ability to create only a single pod containing an infrastructure agent container when combined with kubelet.enabled == false and ksm.enabled == false.

This is useful when you already have your kubernetes infrastructure monitored by newrelic at the infrastructure layer and you want to retrieve the length of some redis keys which are relevant at the application layer.

For more details of the motivation for this change please see #943

Fixes: #943

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature / enhancement (non-breaking change which adds functionality)
• Security fix
• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Add changelog entry following the contributing guide
• Documentation has been updated
• This change requires changes in testing:
  • unit tests
  • E2E tests

I tested this locally and everything works as it ought to

$ helm install -n newrelic-infrastructure tormod ./newrelic-infrastructure/ --set ksm.enabled=false --set controlPlane.enabled=false --set kubelet.agentOnly=false

$ kubectl -n newrelic-infrastructure get all
NAME                             READY   STATUS    RESTARTS   AGE
pod/tormod-nrk8s-kubelet-fblcd   2/2     Running   0          19s
pod/tormod-nrk8s-kubelet-fjn45   2/2     Running   0          19s

NAME                                  DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
daemonset.apps/tormod-nrk8s-kubelet   2         2         2       2            2           <none>          20s

$ kubectl -n newrelic-infrastructure get configmaps
NAME                            DATA   AGE
kube-root-ca.crt                1      22h
tormod-nrk8s-agent-kubelet      1      44s
tormod-nrk8s-integrations-cfg   1      44s
tormod-nrk8s-kubelet            1      44s

$ helm uninstall -n newrelic-infrastructure tormod
release "tormod" uninstalled

$ helm install -n newrelic-infrastructure tormod ./newrelic-infrastructure/ --set ksm.enabled=false --set controlPlane.enabled=false --set kubelet.agentOnly=true

$ kubectl -n newrelic-infrastructure get all
NAME                                      READY   STATUS    RESTARTS   AGE
pod/tormod-nrk8s-agent-7bdd794558-2c9zq   1/1     Running   0          18s

NAME                                 READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/tormod-nrk8s-agent   1/1     1            1           18s

NAME                                            DESIRED   CURRENT   READY   AGE
replicaset.apps/tormod-nrk8s-agent-7bdd794558   1         1         1       18s

$ kubectl -n newrelic-infrastructure get configmaps
NAME                            DATA   AGE
kube-root-ca.crt                1      22h
tormod-nrk8s-agent-kubelet      1      48s
tormod-nrk8s-integrations-cfg   1      48s

$ helm uninstall -n newrelic-infrastructure tormod
release "tormod" uninstalled

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