node-exporter: Add metric for kernel parameters

[Antonios Dimtsoudis]

Hi all,

we are on creating a meric containing the kernel cmdline parameters as labels.

This is e.g. useful in cloud environments, to identify and bundle VMs are booted with different parameters, for example:

# HELP kernel_boot_parameters The kernel boot parameters from /proc/cmdline.
# TYPE kernel_boot_parameters gauge
kernel_boot_parameters{boot_image="(http)/path/to/vmlinuz",version="1.2.3",cloud-init="enabled",...} 1

If you think that is something for upstream, we would be happy to provide a contribution via PR.

Best,
Antonis.