The subject here is
вы. Your sentence is taken from a dialog:
— Я, ваше превосходительство... Очень приятно-с! Друг, можно сказать, детства и вдруг вышли в такие вельможи-с! Хи-хи-с.
— Ну, полно! — поморщился толстый.— Для чего этот тон? Мы с тобой друзья детства — и к чему тут это чинопочитание!
Here the тонкий, Porfiriy, is addresing his schoolmate Misha the толстый with the polite 2nd person plural after having learned his schoolmate has already got a very high rank of Privy Councilor (тайный советник) equal to Lieutenant-General in the Army.
The sentence itself is eliptical, it is a compound sentence consisting of two independent clauses connected with the conjunction
и, and the subject pronouns in both of them are omitted. They can be easily reconstructed, though:
[Вы —] друг … детства и вдруг [вы] вышли в такие вельможи-с!
Друг детства cannot be a direct address, since here Porfiriy is speaking very politely, he uses a словоер (the particle of reverence,
-c at the very end of the sentence), and Privy Councilors could not be addressed with "friend" by those of lower ranks, "Your Excellency" (ваше превосходительство) was the way to address them which Porfiriy does in the next phrase he says.