proposed genserver with new API

final_genserver.ex

defmodule IncrementableValue do
  use StateMapGenServer

  def start_link(config \\ []) do
    config = Keyword.merge([increment_by: 1], config)
    GenServer.start_link(__MODULE__, config, [])
  end

  def init(config) do
    {:ok, %{value: 1}, config}
  end

  def handle_call(:get_data, _from, %{value: value}, _config) do
    {:reply, value} #no need to reply with a new state since nothing is changing
  end

  def handle_cast(:increment, %{value: value}, %{increment_by: increment_by}) do
    {:noreply, %{value: value + increment_by}}
  end
end