curl --request GET \
  --url https://ebn.telemetree.io/public-api/retention \
  --header 'x-api-key: <api-key>'

{
  "data": [
    {
      "cohorts": {},
      "retentionMatrix": [
        [
          "<string>"
        ]
      ],
      "combinedRetention": [
        123
      ],
      "totalUsers": 123,
      "groupBy": "<string>",
      "groups": [
        {}
      ]
    }
  ]
}