Include hugepages metrics in metricbeat

[bczifra]

If metricbeat could capture hugepages metrics such as HugePages_Total, HugePages_Free,HugePages_Rsvd,HugePages_Surp,Hugepagesize, and Hugetlb it would provide a more granular view of memory usage on the system.

system.memory.used.pct includes memory that the system has allocated to huge pages.
e.g.

[ hostname ] # cat /proc/meminfo | egrep "Total|Free|Huge"
MemTotal:       132027020 kB
MemFree:        10274084 kB
…
HugePages_Total:   56274
HugePages_Free:    23506
…
Hugepagesize:       2048 kB

Metricbeat reports this system as having ~92% memory used based on 100-(MemFree/(MemTotal/100)) - however there’s actually 23506*2048kB available as huge pages, which is about 58% of huge pages free.