Было приемлемо и в данном случае не вызвано метрической необходимостью.
First of all, we are talking about poetry, and in Russian poetry this kind of slight modification can be made just because author considered that it would sound nicer, rhythmically or in some other sense nicer.
Second, we are talking here about a piece of text written in the first half of XIX century and Russian, of course, through that period of time went through some changes. You just can not take modern set of rules and apply them to a text written almost two hundred years ago.
This is not the only place in "Горе от ума" where "об" is used where modern speaker would rather expect just "о". Here's incomplete list of similar quotes:
- Уж об твоем ли не радели
- Об воспитаньи! с колыбели!
- Вот об себе задумал он высоко...
- Об Софье Павловне: быть может, нездорова?
- Стучали об пол не жалея!
- Я не об дядюшке об вашем говорю;
In the majority of cases modern Russian speaker indeed would have used "o" but actually even modern speaker in some cases can use "об" with consonant + Prepositional. It can be either an attempt to add some comical effect (like in "об чём тут говорить") or just a regular colloquial usage (like in "об наследстве" or "об вещах"). Even nowadays this list is quite long, and in XIX century set of de-facto acceptable "об"-usages was just bigger.
Pushkin used such form ("Об ней жалели в доме все"), Толстой used it ("он и не думал об Жюли"), Достоевский used it ("моё дело только об вас доложить"), Куприн used it ("сердце Аларина крепко и болезненно билось об грудную клетку) and I can go on and on. In fact, had someone payed me a penny for each example I provide - today I'd be able to have quite a good lunch.