fix: more accurate memory usage statistics

[shawaj]

As per https://psutil.readthedocs.io/en/latest/#psutil.virtual_memory the best way to calculate available and used memory is by using the "total" and "available" metrics.

Also see https://github.com/giampaolo/psutil/blob/master/scripts/meminfo.py

Related to #485 and #522

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Checklist

• Tests added
• Cleaned up commit history (rebase!)
• Documentation added
• Thought about variable and method names

[sonarcloud]

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[shawaj]

@robputt could we also use from psutil._common import bytes2human to simplify this bit as well?

hm-diag/hw_diag/templates/device_info.html

Lines 135 to 147 in 5bc4892

	<td>RAM Usage</td>
	<td class="text-right">
	<div class="progress progress-xl">
	<div class="progress-bar bg-success text-black" role="progressbar" style="width: {{ ((device_metrics.memory_used/device_metrics.memory_total) * 100)|round|int }}%;" aria-valuenow="{{ device_metrics.memory_used|round|int }}" aria-valuemin="0" aria-valuemax="{{ device_metrics.memory_total|round|int }}">{{ (device_metrics.memory_used / 1048576)|round|int }} MB / {{ (device_metrics.memory_total / 1048576)|round|int }} MB</div>
	</div>
	</td>
	</tr>
	<tr>
	<td class="border-0">Disk Usage</td>
	<td class="border-0 text-right">
	<div class="progress progress-xl">
	<div class="progress-bar bg-success text-black" role="progressbar" style="width: {{ ((device_metrics.disk_used/device_metrics.disk_total) * 100)|round|int }}%;" aria-valuenow="{{ device_metrics.disk_used|round|int }}" aria-valuemin="0" aria-valuemax="{{ device_metrics.disk_total|round|int }}">{{ "%.2f"|format((device_metrics.disk_used / 1073741824)|float) }} GB / {{ "%.2f"|format((device_metrics.disk_total / 1073741824)|float) }} GB</div>
	</div>

https://github.com/giampaolo/psutil/blob/7eadee31db2f038763a3a6f978db1ea76bbc4674/psutil/_common.py#L816

[shawaj]

Actually ignore my last comment @robputt as it isn't a number so wouldn't allow us to actually show the progress bar properly I guess.

Although there also appears to be an undocumented percent feature... psutil.virtual_memory().percent

And the same goes for disk usage... psutil.disk_usage('/').percent
https://psutil.readthedocs.io/en/latest/#psutil.disk_usage