query: {
  _aggregation: {
      visits: {
        visit_label: {
          histogram: {
            key
            count
          }
        }
        follow_ups: {
          days_to_follow_up: {
            histogram(rangeStep: 1) {
              key
              count
            }
          }
        }
      }
    }
  }
}

{
  "data": {
    "_aggregation": {
      "subject": {
        "visits": {
          "visit_label": {
            "histogram": [
              {
                "key": "vst_lbl_3",
                "count": 29
              },
              ...
            ]
          },
          "follow_ups": {
            "days_to_follow_up": {
              "histogram": [
                {
                  "key": [
                    1,
                    2
                  ],
                  "count": 21
                },
                ...
              ]
            }
          }
        }
      }
    }
  }
}