"sample" an agent every second?

Just send yourself a message using send_after then from the handler broadcast your state:

 def init(_) do
    # compute init state
    Process.send_after(self(), :tick, 1000)
    {:ok, state}
  end

def handle_info(:tick, state) do
    Phoenix.PubSub.broadcast(My.PubSub, "sample", {:sample, state})
    Process.send_after(self(), :tick, 1000)
end