[3.x] MessageQueue - Fix max usage performance statistic

[lawnjelly]

Fixes to keep track of the maximum usage over time, rather than current usage.

Fixes #76518
Regression from #75527

Notes

• Now the queue is growable / shrinkable, the more interesting statistic is now the current allocated size, but this is left for another PR.
• The statistic here is in theory useful to know you are nowhere near blowing the message queue, but the defaults limits are far higher now after [3.x] Make MessageQueue growable #75527 , so this is unlikely to happen.

[akien-mga]

Thanks!