How to get last_bar_index on bar_index == 0

Question

I want to have on the first bar the bar_index of the last bar available on the chart. The problem is that the last_bar_index is not updated when a new candle starts, ex checking BTCUSDT on 1 min chart

var arr_test_1 = array.new_float()
var arr_test_2 = array.new_float()

if array.size(arr_test_1) > 2
    array.shift(arr_test_1)
array.push(arr_test_1, last_bar_index )


// this is not updated when a new bar starts
if bar_index == 0 
    if array.size(arr_test_2) > 2
        array.shift(arr_test_2)    
    array.push(arr_test_2, last_bar_index )


var label lbl_4 = na
label.delete(lbl_4)
lbl_4 := label.new (barstate.islast ? bar_index  : na, 
     y=close, 
     xloc=xloc.bar_index, 
     yloc=yloc.belowbar, 
     style = label.style_label_up,
     size = size.large,
     color = color.new(color.aqua, 60),
     textcolor = color.new(color.black, 0),
     textalign = text.align_left,
     text = "arr_test_1 " + str.tostring( arr_test_1  )  + "\narr_test_2 " +  str.tostring(  arr_test_2 )  + "\nlast_bar_index " +  str.tostring(  last_bar_index )
     )