Standardize memory metric names

[SuperQ]

In order to compare memory metrics cross-platform, some of the metrics need to be renamed where they are appropriately comparable.

Some candidates that seem to match on all platforms:

node_memory_bytes_total
node_memory_free_bytes_total

[brian-brazil]

I'm not sure this is sensible, to be comparable and thus to have the same name they should have exactly the same meaning and I'm betting that's not the case for even those two metrics on the platforms we support.

[discordianfish]

Tried that, was quite painful. For reference: #233

[stuvie]

standardizing the metrics will allow us to have cross-platform grafana dashboards

[SuperQ]

@stuvie The problem is that cross-platform dashboards are likely to generate incorrect results.

For Grafana, cross-platform views can be generated by having multiple queries in a single graph that return the correct results based on the platform in question.

[SuperQ]

Since we've decided that it's not really comparable between platforms, I will close this.