I can certainly help you out with the key file and service account email. Google is really good at changing their documentation and procedures often and usually not in that order.
Create your project in the Google console: https://code.google.com/apis/console
Go to API's & Auth and turn off all but the Analytics API.
Go to Credentials (under API's) and click Create a New Client ID.
Select Service Account from the pop up.
Save the .p12
file it will prompt you to download.
The service account email address will be under the Service Account setting box as Email Address and be in the form of xxxxxxxxxxxxxx@developer.gserviceaccount.com (the really long email)
Add this email to the Google analytics profile(s) as a user with Read & Analyze access.
You should now have all the pieces you need to use service account access once you find that Rails tut.
Setting up OAuth 2
As far as you Rails app goes, I found this (you may have as well): Google Analytics API Client Library for Ruby
HTH