This blog post describes how to get credentials for using Googles Analytics Core Reporting API with module for the MagicMirror².
See the full setup and capabilities of this Magic Mirror Module on Github: MMM-GoogleAnalytics.

1. Create a Service Account Googles Developer console

• First create a new project on Googles Developer console like “mywebsiteGAapi”.
• Under the section “Credentials”, execute “Create credentials” and choose “Service account key
  
• Create a new service account and choose a Service account name
  
  • Role: Project -> Viewer
  • Key type: JSON
  • Copy the email address under Service account ID
• Click “Create”, save and rename your credential file (e.g. mywebsiteGAapi-6116b1dg49a1.json) in keyfile.json and save it in your MMM-GoogleAnalytics folder., like MMM-GoogleAnalytics/keyfile.json

2. Create a User and get your View ID from your Google Analytics Account

• open the Admin panel in your Google Analytics Account and enter the User Management section
  
• Create a new user
  • Email address:  The one you copied previously (It’s also in the JSON-file you already saved)
  • Property Permissions: Read & Analyse
• Back in the Admin panel under View enter the View Settings
  
  
• Copy the View ID
  

Perfect you did it, that’s all!