Improve display of used/cached memory in Infra monitoring

[Berdir]

Is your feature request related to a problem?

The infra dashboard overview but also the detail graphs do not show memory usage in a useful way

Several of my servers report a high/orange to very high/red memory usage:

But looking at the detailed graph for e.g. the 94% server shows that a large chunk of that is just cached:

that the operating system uses memory to cache stuff is good, this is not an unhealthy situation.

Describe the solution you'd like

The overview should subtract cached from the graph. This is also how the panel from the old host metrics dashboard works:

The detail page could maybe be represented in a different way that makes it easier to see real usage vs. cached, maybe a stacked graph with lighter colors for cached/buffer?

(cpu idle is somewhat similar, it seems to add little value in practice, I'd rather see a stacked graph of user + system + wait +..)

Describe alternatives you've considered

Additional context

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