improve mkr status -v to show host metrics

[kmuto]

This patch adds to mkr the ability to output a list of host-metrics that the host has.
The output of the metric list will only work with mkr status -v (verbose mode), so there will be no performance impact in general use.

$ mkr status 4Hkc5RWzUQG
{
    "id": "4Hkc5RWzUQG",
  ...
    "ipAddresses": {
 ...
    }
}

$ mkr status -v 4Hkc5RWzUQG
{
    "id": "4Hkc5RWzUQG",
  ...
    "interfaces": [
 ...
    ],
    "metrics": [
        "cpu.guest.percentage",
 ...
        "memory.used"
    ]
}