"Leaker"? No !
"Comey admitted he became a leaker – giving his notes to a friend to pass to journalists"
This is NOT a leak. Comey was a party to the conversation and is entitled to report it however he wants (excluding obvious exception such as classified info, or legal advice, etc that don't apply here).
If it had been someone else's conversation that he'd heard then that would be a leak. But if it's his own then he owns it (jointly with Trump, I guess) and he's free to report it.