in src/main/scala/com/gu/fastly/api/FastlyApiClient.scala [209:213]
def statsUsage(from: DateTime, to: DateTime, by: By.Value, region: Region.Value = Region.all): Future[Response] = {
val apiUrl = s"$fastlyApiUrl/stats/usage"
val params = statsParams(from, to, by, region)
AsyncHttpExecutor.execute(apiUrl, headers = commonHeaders, parameters = params)
}