[Monitoring] Report total, free, and available memory metrics

[andrewkroh]

Describe the enhancement:

Today beats metrics only report the RSS metric for the process. It would be more useful if it were accompanied by other information about the host's memory including total, used, free, and available memory.

Describe a specific use case for the enhancement or feature:

When reviewing metrics recorded in diagnostic snapshots, it would be useful to have some context about how much of the system's memory is being used to help give context about the process's RSS value. Having this context could help identify if the process is under memory pressure.

[botelastic]

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