Mix Panel Proxy to stop ad blockers while still allowing tracking. Transliterated from https://github.com/mixpanel/flask-tracking-proxy

mixpanel_proxy_controller.ex

defmodule HelloWeb.MixPanelProxyController do
  use HelloWeb, :controller

  @excluded_headers [
    "content-encoding",
    "content-length",
    "transfer-encoding",
    "connection",
    "access-control-allow-credentials",
    "access-control-allow-origin"
  ]

  def js_lib(conn, _) do
    handle_api_request(conn, "https://cdn.mxpnl.com/libs/mixpanel-2-latest.js")
  end

  def js_lib_minified(conn, _) do
    handle_api_request(conn, "https://cdn.mxpnl.com/libs/mixpanel-2-latest.min.js")
  end

  def api_request(conn, params) do
    %{"path" => [path | _]} = params

    mixpanel_url =
      if path == 'decide', do: "https://decide.mixpanel.com", else: "https://api.mixpanel.com"

    # if not using fly.io you'll need to change this
    ip = conn |> get_req_header("Fly-Client-IP")

    case HTTPoison.request(%HTTPoison.Request{
           method: conn.method,
           url: mixpanel_url <> "/" <> path,
           params: conn.query_params,
           headers: %{"x-real-ip" => List.first(ip, "0.0.0.0")},
           body: {:form, Map.to_list(conn.body_params)}
         }) do
      {:ok, %HTTPoison.Response{} = response} ->
        conn
        |> merge_resp_headers(response.headers)
        |> filter_headers(@excluded_headers)
        |> send_resp(response.status_code, response.body)

      {:error, %HTTPoison.Error{} = error} ->
        conn |> put_status(400) |> json(%{"error" => inspect(error)})
    end
  end

  defp handle_api_request(conn, url) do
    case HTTPoison.get(url) do
      {:ok, %HTTPoison.Response{} = response} ->
        conn
        |> merge_resp_headers(response.headers)
        |> filter_headers(@excluded_headers)
        |> send_resp(response.status_code, response.body)

      {:error, %HTTPoison.Error{}} ->
        conn |> send("error")
    end
  end

  defp filter_headers(conn, [first_header | remaining_headers]) do
    Plug.Conn.delete_resp_header(conn, first_header)
    |> filter_headers(remaining_headers)
  end

  defp filter_headers(conn, []), do: conn
end

router.ex

defmodule VeloaWeb.Router do
  ....
  pipeline :mixpanel do
    plug(:put_secure_browser_headers)
    plug Corsica, origins: "*", allow_credentials: true
  end

  scope "/mixpanel", VeloaWeb do
    pipe_through([:mixpanel])
    get("/lib.js", MixPanelProxyController, :js_lib)
    get("/lib.min.js", MixPanelProxyController, :js_lib_minified)
    get("/*path", MixPanelProxyController, :api_request)
    post("/*path", MixPanelProxyController, :api_request)
  end
end