[collectd 6] gpu_sysman: use base units + other metric handling fixes

[eero-t]

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ChangeLog: gpu_sysman: use base units + other metric handling fixes

This split the fixes from #4267 to their own PR (that should go in first).

Changes are IMHO straightforward and easiest to review from individual commits.

(Because switching to base units means metric names changing, Feature label could have been used instead of a Fix one.)