Дмитрий

lastCarpetId { value ->
            //value получен
        }

fun lastCarpetId(callback: (String) -> Unit)  {
        val db: FirebaseFirestore = FirebaseFirestore.getInstance()
        db.collection("carpets")
            .orderBy("carpetid")
            .limit(1)
            .get()
            .addOnCompleteListener {
                if (it.isSuccessful) {
                    for (document in it.result!!) {
                        callback(document.data.getValue("carpetid"))
                    }
                }
            }
    }