At this point this is what kea is sending back:

{
    "command": "statistic-get-all",
    "arguments": {
        "declined-addresses": [ [ 0, "2019-07-30 10:04:28.386733" ] ],
        "reclaimed-declined-addresses": [ [ 0, "2019-07-30 10:04:28.386735" ] ],
        "reclaimed-leases": [ [ 0, "2019-07-30 10:04:28.386736" ] ],
        "subnet[1].assigned-addresses": [ [ 0, "2019-07-30 10:04:28.386740" ] ],
        "subnet[1].declined-addresses": [ [ 0, "2019-07-30 10:04:28.386743" ] ],
        "subnet[1].reclaimed-declined-addresses": [ [ 0, "2019-07-30 10:04:28.386745" ] ],
        "subnet[1].reclaimed-leases": [ [ 0, "2019-07-30 10:04:28.386747" ] ],
        "subnet[1].total-addresses": [ [ 200, "2019-07-30 10:04:28.386719" ] ]
    },
    "result": 0
}

I want to focus on a prat with subnets, which is really hard to parse. Biggest issue is that subnet id is in key name, not as value. And this is completely flat.

I am proposing to return statistics like that:

{
    "command": "statistic-get-all",
    "arguments": {
        "declined-addresses": [ [ 0, "2019-07-30 10:04:28.386733" ] ],
        "reclaimed-declined-addresses": [ [ 0, "2019-07-30 10:04:28.386735" ] ],
        "reclaimed-leases": [ [ 0, "2019-07-30 10:04:28.386736" ] ],
        "subnets": { [
            "subnet-id": 1,
            "assigned-addresses": [ [ 0, "2019-07-30 10:04:28.386740" ] ],
            "declined-addresses": [ [ 0, "2019-07-30 10:04:28.386743" ] ],
            "reclaimed-declined-addresses": [ [ 0, "2019-07-30 10:04:28.386745" ] ],
            "reclaimed-leases": [ [ 0, "2019-07-30 10:04:28.386747" ] ],
            "total-addresses": [ [ 200, "2019-07-30 10:04:28.386719" ] ]
           ],
           [
            "subnet-id": 2,
            "assigned-addresses": [ [ 0, "2019-07-30 10:04:28.386740" ] ],
            "declined-addresses": [ [ 0, "2019-07-30 10:04:28.386743" ] ],
            "reclaimed-declined-addresses": [ [ 0, "2019-07-30 10:04:28.386745" ] ],
            "reclaimed-leases": [ [ 0, "2019-07-30 10:04:28.386747" ] ],
            "total-addresses": [ [ 200, "2019-07-30 10:04:28.386719" ] ]
           ]
        }
    },
    "result": 0
}

I came up on this issue while working recently with performance testing of a setup with ~500 subnets.