Kubernetes examples and documentation should store state on a host path volume

[cmacknz]

All examples and documentation for deploying the Elastic agent on Kubernetes should mount the state path on a host path volume to prevent loss of persistent state like the Filebeat registry when a pod is recreated.

When the agent is run on ECK this is done by default: https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-elastic-agent-configuration-examples.html#k8s_storing_local_state_in_host_path_volume

Specifically the the directory we want to store on a host path volume is the configured STATE_PATH. The STATE_PATH directory was introduced in elastic/beats#24817 and should also be documented in the official documentation in https://github.com/elastic/observability-docs/tree/main/docs/en/ingest-management

[cmacknz]

Closed by #2550