fix: weighted average calculation for consumption in drives dashboard

[swiffer]

the average consumption in drives dashboard should be a weighted average, not a simple mean of consumption

@jheredianet - beeing the original author of the top level stats. what do you think? please check.

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[swiffer]

@JakobLichterfeld approved by jheredianet by thumbs up ;) could you quickly verify in grafana image and merge before release? Thank you!!!

[JakobLichterfeld]

@JakobLichterfeld approved by jheredianet by thumbs up ;) could you quickly verify in grafana image and merge before release? Thank you!!!

Ok, we do count that :-)

Successfully tested on my end.