mysql trigger to keep track of how often a column was updated

I have a column "processed_at" on table. This can get reset from multiple places in the code in order to indicate to a job that this row needs to be processed. I would like to find out how processed_at is set to null.

What is the easiest way to do this? Ideally I would know how often this happens by row id, but it would also be ok to just know a number for all rows combined over a certain period.

Can this be done like this:
A trigger that reacts to the update and then stores id and reset timestamp to a separate table?
Would this have a noticeable effect on the performance of the original query?

Something like this:

create table mytable_resets (
  id serial primary key,
  mytable_id bigint unsigned not null,
  reset_at datetime not null
);

delimiter ;;

create trigger t after update on mytable
for each row begin
  if NEW.processed_at is null then
    insert into mytable_resets values (default, NEW.id, NOW());
  end if;
end;;

delimiter ;

Yes, it will impact the performance of the original query.

The cost of database writes is roughly proportional to the number of indexes it updates. If your query execute a trigger to insert into another table, it adds another index update. In this case, the primary key index of the mytable_resets table.

But it shouldn't be significantly greater overhead than if your mytable table had one more index.