Add "metadata" section to json output

[forkwhilefork]

For example, NTT's published json feed contains the following:

    "metadata": {
        "buildmachine": "rpkiv00.dllstx09.us.to.gin.ntt.net",
        "buildtime": "2023-02-27T01:36:13Z",
        "elapsedtime": "258",
        "usertime": "239",
        "systemtime": "21",
        "roas": 141242,
        "failedroas": 4,
        "invalidroas": 0,
        "aspas": 2,
        "failedaspas": 0,
        "invalidaspas": 0,
        "bgpsec_pubkeys": 3,
        "certificates": 35892,
        "invalidcertificates": 0,
        "taks": 0,
        "tals": 5,
        "invalidtals": 0,
        "talfiles": [
            "/etc/pki/tals/arin.tal",
            "/etc/pki/tals/ripe.tal",
            "/etc/pki/tals/lacnic.tal",
            "/etc/pki/tals/apnic.tal",
            "/etc/pki/tals/afrinic.tal"
        ],
        "manifests": 35889,
        "failedmanifests": 54,
        "stalemanifests": 0,
        "crls": 35835,
        "gbrs": 2,
        "repositories": 83,
        "vrps": 408848,
        "uniquevrps": 404513,
        "vaps": 20,
        "uniquevaps": 10,
        "cachedir_del_files": 24,
        "cachedir_superfluous_files": 7998,
        "cachedir_del_dirs": 2071
    },

And Cloudflare's contains the following:

"metadata": {
    "counts": 408822,
    "generated": 1677469609,
    "valid": 1677473209,
    "signature": "3045022100cc935dc6ee4b54bb82501c684c399d360ffe36afbc3f5ffa6a02dce37b3e3383022053c199d80b47c95410c2f2eb95a17baa9660fd0ce0308b86b4102fffc2949b88",
    "signatureDate": "3045022100f45030d4adc64eb79565f5545c40857dc2934919e17871c9a38b391abf67e926022077d83532707e2b9b9b26dd784ef12b0f53f4a93c1e4cf7d8c6b340b3b6c1c35a"
  }

It would be nice to have, at a minimum, the date/time the file was generated.