[k8s] System Integration Does Not Utilise /hostfs Mounts

[pkoutsovasilis]

Version:
All current versions of Elastic Agent Helm Chart and old k8s manifests.

Operating System:
Kubernetes

Steps to Reproduce:

1. Deploy the Elastic Agent using either the Helm Chart or the old manifests with default configurations.
2. Verify that /proc and /sys are mounted under /hostfs.
3. Observe that the system integration is not configured to utilise these mounts, leading to the monitoring of only processes visible inside the container.

Description of the Issue:
The Elastic Agent system integration does not correctly utilise the mounted /hostfs paths (/proc and /sys) in the default configurations of both the Helm Chart and the old k8s manifests. As a result, it only monitors the processes within the container, instead of the intended host-level metrics. This issue results in incomplete or incorrect data streams.

Credits:
This issue was identified by @swiatekm