How to authenticate to Google Analytics Reporting API v4

Found out what I was missing :

• Google APIs Client Library "Options" :

  google.options({ auth: oauth2Client }); //this one is not very optional
  

• Unlike Google Analytics Reporting API v4 documentation, queries using the client library must have headers to specify a client for each requests (thanks to CVarisco who notice that, client library documentation isn't really accurate..) :

  var request ={
      'headers': {'Content-Type': 'application/json'},
      'auth': oauth2Client,
      'resource': req,
  };