You already talked to a lawyer that told you not to report it, but you ignored them anyway and disclosed it. At this point there's a record within your medical school of disclosure, so it would be most consistent to simply report it for residency too. What wording does the residency application use for criminal record? If it says "have you ever been arrested and if so why?" then yes you probably need to be forthcoming about being charged with alcohol and then being reduced to littering. If it just says "have you ever been convicted of a crime and if so what?" then you probably can say just littering. Nevertheless, be consistent with what you wrote in your medical school application.
With the medical board, you will have to disclose this for sure and may likely delay you in getting the license quickly as they'll probably ask you to send them court documents. I would consider hiring a lawyer so they can get all these for you, unless you know you can get all this stuff easily on your own
Quite honestly this is such a petty thing that I'd be surprised if anyone cared about this charge/conviction
PS. Just personal advice. I'm not a lawyer or intending to give legal advice