Diagnostics should include a summary of monitored pods when running on Kubernetes

[cmacknz]

The Elastic Agent diagnostics should include a Kubernetes specific report about the number of containers it is monitoring, along with some basic information about those containers. We should include the container image, the resource name, namespace, and resource kind of each container the agent is monitoring.

Kubernetes environments are dynamic and when autodiscover is enabled the set of pods running changes constantly. Our diagnostics bundles need to include a snapshot of the current running containers along with basic information about them.

We should also log the number of monitored containers periodically to give us a way to more easily observe changes in the Kubernetes cluster purely from our own logs. For example, resource spikes in the agent frequently correlate with new deployments in the cluster when autodiscover is enabled. We need a way to see this.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)