ui: fix zero memory utilization bug on systems using cgroups v2

[shoenig]

A quick band-aid fix for #13023. On systems using cgroups v2, the MemoryStats struct no longer reports .RSS, but rather .Usage. Kind of just assuming they mean the same thing.

The fix here is to just check if .Usage exists and if so use that, otherwise fallback to the .RSS field.

"MemoryStats": {
	"RSS": 0,
	"Cache": 0,
	"Swap": 0,
	"MappedFile": 0,
	"Usage": 98656256,
	"MaxUsage": 0,
	"KernelUsage": 0,
	"KernelMaxUsage": 0,
	"Measured": [
		"Cache",
		"Swap",
		"Usage"
	]
},

@philrenaud @ChaiWithJai

Fixes #13023

[github-actions]

Ember Asset Size action

As of d1e14f1

Files that got Bigger 🚨:

File	raw	gzip
nomad-ui.js	+84 B	+10 B

Files that stayed the same size 🤷‍:

File	raw	gzip
vendor.js	0 B	0 B
nomad-ui.css	0 B	0 B
vendor.css	0 B	0 B

[github-actions]

Ember Test Audit comparison

	main	d1e14f1	change
passes	1380	1380	0
failures	2	2	0
flaky	0	0	0
duration	000ms	000ms	-000ms

[shoenig]

Gave this a shot but I don't know how to make this test happy
https://app.circleci.com/pipelines/github/hashicorp/nomad/28941/workflows/9f8927bd-e506-4ea1-9fa0-03e76adb2927/jobs/320070

[philrenaud]

Thanks @shoenig , digging into that test now

[github-actions]

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