Add system metrics reporting total memory capacity or clarify how to recover existing ones

[mx-psi]

Current system metrics cover usage of memory, paging/swap memory and filesystems, but we don't currently support the total capacity of any of these systems as a separate metric.

The following metric can't be recovered from the existing system metrics and would need to be added to support this:

Name	Description	Units	Instrument Type	Value Type	Attribute Key	Attribute Values
system.disk.limit	Total memory available in the disk.	By	UpDownCounter	Int64	device	(identifier)

The following metric may be recovered from system.memory.usage, but the current description of the attribute values of state is insufficient to recover this.

Name	Description	Units	Instrument Type	Value Type	Attribute Key	Attribute Values
system.memory.limit	Total memory available in the machine. Does not include paging/swap memory.	By	UpDownCounter	Int64	n/a	n/a

The following metrics are available as the sum of used and free (and reserved for the file system one). They could be added as a convenience metric:

Name	Description	Units	Instrument Type	Value Type	Attribute Key	Attribute Values
system.filesystem.limit	Total memory available in the disk.	By	UpDownCounter	Int64	device	(identifier)
					state	used, free, reserved
					type	ext4, tmpfs, etc.
					mode	rw, ro, etc.
					mountpoint	(path)
system.paging.limit	Total paging/swap memory available.	By	UpDownCounter	Int64	n/a	n/a

Items for this issue:

• [ ] Add system.disk.limit to the specification
• [x] Clarify set of attribute values for system.memory.usage state and consider using system.memory.total
• [ ] Consider adding system.filesystem.limit and system.paging.total if convenience justifies it

This would be part of open-telemetry/opentelemetry-specification/issues/3556 if approved.