For those who are looking for aggregate usage by user (perhaps for monetization) - there is a "byUser" endpoint as well. The request is structured like below:
https://{api-service-name}.management.azure-api.net/subscriptions/{subscription}/resourceGroups/{resource-group}/providers/Microsoft.ApiManagement/service/{api-service-name}/reports/byUser?$filter=timestamp ge datetime'2019-12-01T00:00:00' and timestamp le datetime'2019-12-04T00:00:00'&api-version=2017-03-01
The documentation says to make a request to "https://management.azure-api.net/[...]" but I had to prepend the resource name like in the request above.
And the response:
{
  "value": [
    {
      "name": "Administrator",
      "userId": "/users/1",
      "callCountSuccess": 13,
      "callCountBlocked": 1,
      "callCountFailed": 0,
      "callCountOther": 0,
      "callCountTotal": 14,
      "bandwidth": 11019,
      "cacheHitCount": 0,
      "cacheMissCount": 0,
      "apiTimeAvg": 1015.7607923076923,
      "apiTimeMin": 330.3206,
      "apiTimeMax": 1819.2173,
      "serviceTimeAvg": 957.094776923077,
      "serviceTimeMin": 215.24,
      "serviceTimeMax": 1697.3612
    },
    {
      "name": "Samir Solanki",
      "userId": "/users/56eaec62baf08b06e46d27fd",
      "callCountSuccess": 0,
      "callCountBlocked": 0,
      "callCountFailed": 0,
      "callCountOther": 0,
      "callCountTotal": 0,
      "bandwidth": 0,
      "cacheHitCount": 0,
      "cacheMissCount": 0,
      "apiTimeAvg": 0,
      "apiTimeMin": 0,
      "apiTimeMax": 0,
      "serviceTimeAvg": 0,
      "serviceTimeMin": 0,
      "serviceTimeMax": 0
    },
    {
      "name": "Anonymous",
      "userId": "/users/54c800b332965a0035030000",
      "callCountSuccess": 0,
      "callCountBlocked": 0,
      "callCountFailed": 0,
      "callCountOther": 0,
      "callCountTotal": 0,
      "bandwidth": 0,
      "cacheHitCount": 0,
      "cacheMissCount": 0,
      "apiTimeAvg": 0,
      "apiTimeMin": 0,
      "apiTimeMax": 0,
      "serviceTimeAvg": 0,
      "serviceTimeMin": 0,
      "serviceTimeMax": 0
    }
  ],
  "count": 3,
  "nextLink": ""
}
If you need to filter by type of request or API, you can do that as well - List by User