Machine Gun Kelly -- Thanks, Judge ... For Hiding My Boozy Past

Machine Gun Kelly parties HARD ... but his legendary tales of getting wasted can't be used against him when he goes to court for allegedly slicing open a guy's finger during a nasty bar brawl.

Kelly's being sued by a bouncer who claims the rapper nearly took his digit off cleanly ... when he fired a liquor bottle at him during a massive fight at a St. Petersburg, FL tavern in 2012.