Fix GH-16932 Scoreboard reset at end of request

[lucasnetau]

Scoreboard was updated with FPM_SCOREBOARD_ACTION_SET in PR#14153 causing all metrics to be reset at the end of request. Use FPM_SCOREBOARD_ACTION_INC instead to ensure we only update peak memory

Fixes #16932

[bukka]

I just created a PR that includes a test. I would normally comment here and try to guide you to create the test but 8.4.2 will be branched early next week and I really wanted to get it there. I will for sure give you a credit in NEWS (changelog) and will also add Co-authored-by so you get some credit.

I actually chose keeping the SET because I think it's semantically better (the memory is actually being set - I know that it's set in any case but seeing INC might be a bit more confusing).

So closing in favour of #16974

[lucasnetau]

No problems @bukka, thanks for doing the test, I didn't know my way around the fpm tester, will remember that in the future. Fine with keeping at SET, semantically it makes more sense than INC, I went with the minimal change approach when making the PR.