curl --request POST \
  --url 'https://api.flashcat.cloud/report/oncall/changes?app_key=' \
  --header 'Content-Type: <content-type>' \
  --data '
{
  "step": "day",
  "start_time": 123,
  "end_time": 123
}
'

{
  "error": {
    "code": "<string>",
    "message": "<string>"
  },
  "data": {
    "items": [
      {
        "ts": 123,
        "change_cnt": 123,
        "change_event_cnt": 123
      }
    ]
  }
}