Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Here are the facts of the issue,
Even if you know your gamertag, Xbox support wont tell you the email account associated with the gamertag.
So the question is, do you know the email account that your old gamertag MetroLemmings651 was associated with?
If you do know the email address, then try to sign into that email address on this specific page.
Try to sign in using the registered email address and the microsoft account password. If you cant sign in, use the "forgot password" link on the next page.
Once you have the password ***** can sign in.. Now you just need to add that email account to the new xbox,heres how.
Note, you can have more then one gamertag on an xbox..
If you get stuck or run into any issues, let me know! And if all goes well please remember to return here and click and submt a 5 star rating to show I was able to assist you.