add MHz label for node_cpu_info

[silenceshell]

feature requests:

Our machines run on centos 7, and we close p-state to gain max performance. So on these machines, we cannot get node_cpu_frequency_hertz, as the freq module is not loaded.

# ls /sys/devices/system/cpu/cpu0
cache  crash_notes  crash_notes_size  driver  firmware_node  hotplug  microcode  node0  power  subsystem  thermal_throttle  topology  uevent
# lsmod|grep freq
#

We could get cpu info from node_cpu_info:

node_cpu_info{cachesize="15360 KB",core="0",cpu="0",endpoint="https",family="6",instance="1.1.1.1:9100",job="node-exporter",microcode="0x428",model="62",model_name="Intel(R) Xeon(R) CPU E5-2620 v2 @ 2.10GHz",namespace="lambda",package="0",pod="node-exporter-2ntkr",service="node-exporter",stepping="4",vendor="GenuineIntel"}

However, as we run on performance mode, the cpu's frequency is actually 2999.970 (not 2.10GHZ):

# cat /proc/cpuinfo | grep 'MHz'
cpu MHz		: 2999.970

So, is it possible to add a label something like mhz to show the cpu's frequency?
I would like to implement this feature if possible.