Memory usage disagrees with Prometheus

Summary

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/admin/system_info/show.html.haml#L20 results in different value than % memory usage command shown at https://docs.gitlab.com/ee/administration/monitoring/prometheus/#viewing-performance-metrics.

Originally mentioned at https://gitlab.com/gitlab-org/gitlab-ce/commit/085e1f89f327ad4d679696fe4673d09e1840b236#note_84021481.

What is the current bug behavior?

Admin panel and prometheus should give different values

What is the expected correct behavior?

Admin panel and prometheus should give same values

Possible fixes

Same logic used for both. Additionally both may be wrong as other places say to use (1 - ((node_memory_MemFree + node_memory_Buffers + node_memory_Cached) / node_memory_MemTotal)) * 100

/cc @smcgivern