Add OS mem stats to node-stats telemetry device

[b-deam]

With this commit we add the OS memory stats to default list of
collected node-stats metrics.

This adds the below fields to the resulting metrics docs:

[...]
    "os_mem_total_in_bytes": 68719476736,
    "os_mem_adjusted_total_in_bytes": 68719476736,
    "os_mem_free_in_bytes": 260407296,
    "os_mem_used_in_bytes": 68459069440,
    "os_mem_free_percent": 0,
    "os_mem_used_percent": 100,
[...]

[pquentin]

This works beautifully, thank you!