[receiver/hostmetrics] fix filesystem scraper root_path support

[povilasv]

Description

Fixes #35990

Looks like filesystem scraper stopped respecting root_path

The issue it seems to be that the default value in this map is wrong:

var gopsutilEnvVars = map[common.EnvKeyType]string{
	common.HostProcEnvKey:    "/proc",
	common.HostSysEnvKey:     "/sys",
	common.HostEtcEnvKey:     "/etc",
	common.HostVarEnvKey:     "/var",
	common.HostRunEnvKey:     "/run",
	common.HostDevEnvKey:     "/dev",
	common.HostProcMountinfo: "",
}

and this function fails to correctly set envvars:

func setGoPsutilEnvVars(rootPath string, env environment) common.EnvMap {
	m := common.EnvMap{}
	if rootPath == "" || rootPath == "/" {
		return m
	}

	for envVarKey, defaultValue := range gopsutilEnvVars {
		_, ok := env.Lookup(string(envVarKey))
		if ok {
			continue // don't override if existing env var is set
		}
		m[envVarKey] = filepath.Join(rootPath, defaultValue)
	}
	return m
}

Gops utils docs:

You can set an alternative location to /proc/N/mountinfo by setting the HOST_PROC_MOUNTINFO environment variable.

Ref: https://github.com/shirou/gopsutil/blob/6be0508aeb80711cfb45a829590fb4b2b9972bba/disk/disk_linux.go#L260-L261

Testing

Tested in local computer

Documentation

[povilasv]

cc @atoulme

[atoulme]

We don't set env vars anymore, we use the envMap and pass it as part of context. The function name is misleading.

[atoulme]

I have provided a solution with #36000 which I believe fixes the issue. Please take a look.