Revert dup cpu frequency, add node_cpufreq flag (#3795)

Both cpu_linux and cpufreq export cpu frequencies under the same name.
This makes the metric mutually exclusive.

A previous PR made it possible to enable both which can cause flapping
help texts.

This PR re-introduces mutual-exclusive behavior for the frequencies.
A new flag `collector.cpufreq.enable-cpufreq-prefix` can be used to
migrate all cpufreq metrics to node_cpufreq to avoid the collision.

Signed-off-by: René Treffer <treffer@measite.de>