Modify how memory usage is calculated

[cobrul]

trafficstars

Currently, the exporter is showing different values compared to docker stats if the docker daemon runs on linux. In order to prevent misinterpretations, memory usage should be calcuclated like the docker stats command line. See https://github.com/moby/moby/blob/eb131c5383db8cac633919f82abad86c99bffbe5/cli/command/container/stats_helpers.go#L175

[aleksa2808]

Ran into this issue as well. Very interested if this can be merged since, apart from the memory info not being very useful to us, this is a great exporter!

[RalfHerzog]

I noticed that the memory usage reported by the exporter including your PR (thanks btw) is different to the usage reported by docker stats. It seems that a multiplication by 10 delivers the same value. Can you confirm that?