I have a table like so:

ColA ColB
A 1
B 1
C 1
D 2
A 3
B 2
C 3

Is there a formula that I can place in ColA that will count the unique
cells, even when cColB is filtered

For example, if ColB is not filtered, the formula would count 4 unique cells
(A,B,C,D)

If ColB were filtered to 1, the formula would count 3 unique cells (A,B,C).

Thank you in advance.