in_node_exporter_metrics: add support of `path.rootfs` configuration

[vchirikov]

Is your feature request related to a problem? Please describe.

Hi, I want to replace node_exporter with fluent-bit in my k8s cluster (because I already use fluent-bit for logs).
I read the docs and didn't find the path.rootfs parameter which is used in my node_exporter DaemonSet.

If you start container for host monitoring, specify path.rootfs argument.
This argument must match path in bind-mount of host root. The node_exporter will use
path.rootfs as prefix to access host filesystem.

From docs of node_exporter

I searched across the repo and find only path.procfs / path.sysfs which seems not enough to configure properly.

fluent-bit/plugins/in_node_exporter_metrics/ne.h

Lines 51 to 52 in 7e7433c

	flb_sds_t path_procfs;
	flb_sds_t path_sysfs;

I also search in issues and find bot-closed #8434 issue about it from the past.

Without path.rootfs node exporter looks not on the host, but inside container fs.

Describe the solution you'd like

I think fluent-bit should support missed path.rootfs option to have the correct config of in_node_exporter_metrics plugin.

[vchirikov]

Ok, I tried without path.rootfs and even then the plugin doesn't work.

[2025/07/26 12:37:34] [error] [input:node_exporter_metrics:node_exporter_metrics.0] systemd init failed
[2025/07/26 12:37:34] [error] [input:node_exporter_metrics:node_exporter_metrics.0] could not set systemd collector for Node Exporter Metrics plugin

I don't want to even run systemd collector and can't disalbe it! (it's enabled by default)

Found another issues/discussion and it looks like this fluent-bit plugin is completely unusable inside k8s. sad.