Add cpuinfo_avg_freq to list of CPU frequency metrics

[mrobbert]

cpuinfo_avg_freq was recently added as an optional sysfs attribute to the Linux kernel and I would like for it to be exposed in this library for use in node_exporter. Here is the information on its introduction - https://lkml.iu.edu/2501.3/07185.html

Here is what it looks like on my openSUSE Leap 15.6 system

# ls -l /sys/devices/system/cpu/cpu0/cpufreq/
total 0
-r--r--r-- 1 root root 4096 May 20 11:15 affected_cpus
-r--r--r-- 1 root root 4096 May 20 11:15 cpuinfo_avg_freq
-r--r--r-- 1 root root 4096 May 12 10:25 cpuinfo_max_freq
-r--r--r-- 1 root root 4096 May 12 10:25 cpuinfo_min_freq
-r--r--r-- 1 root root 4096 May 12 15:15 cpuinfo_transition_latency
-r--r--r-- 1 root root 4096 May 12 15:15 related_cpus
-r--r--r-- 1 root root 4096 May 12 15:15 scaling_available_governors
-r--r--r-- 1 root root 4096 May 12 10:25 scaling_cur_freq
-r--r--r-- 1 root root 4096 May 12 15:15 scaling_driver
-rw-r--r-- 1 root root 4096 May 12 15:15 scaling_governor
-rw-r--r-- 1 root root 4096 May 12 15:15 scaling_max_freq
-rw-r--r-- 1 root root 4096 May 12 15:15 scaling_min_freq
-rw-r--r-- 1 root root 4096 May 12 15:15 scaling_setspeed

# cat /sys/devices/system/cpu/cpu0/cpufreq/*
0
1456572
3700000
1000000
0
0
performance powersave
1000000
intel_pstate
powersave
3700000
1000000
<unsupported>

I think that this would be a simple change so I can provide a PR if you'd like, but maybe it is easier for an existing developer to add it.