What does this PR do?

Always use the local filesystem for self-monitoring metrics.
Metricbeat will use the hostfs to set the environment variables:

• HOST_PROC
• HOST_SYS
• HOST_ETC

which are read by github.com/shirou/gopsutil/v4 and used when fetching metrics. However, the hostfs is not available by the time self monitoring metrics are initialised, so when running in a container environment, we cannot resolve the PID relative to the hostfs PID namespace.

This causes elastic-agent-system-metrics to get the PID using os.Getpid, and gopsutil uses the hostfs to fetch all metrics, causing us to get metrics from the wrong process or getting an error because /hostfs/proc/<PID>/fd does not exist.

Always using the local filesystem for self-monitoring metrics solves this problem.

Why is it important?

It allows a Beat to correctly get metrics from its own process.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

## Author's Checklist

Related issues

• Metricbeat container throws errors when fetching metrics with hostfs set beats#48690