Migrate + enhance a script from Google Sign-In JavaScript to to the new Google Identity Services

We have a webpage using Google Sign-In JavaScript to login/identify into an interface.
2 tasks to be done:
1. migrate from Google Sign-In JavaScript to the new Google Identity Services
2. Enhance: allow a list of registered users ie: email1, email2
3. enhance the security of the script by not having the API_KEY visible


Script Extract:

function initClient() {
var API_KEY='xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
var CLIENT_ID= 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com';
var SCOPE = 'https://www.googleapis.com/auth/spreadsheets.readonly';

gapi.client.init({
'apiKey': API_KEY,
'clientId': CLIENT_ID,
'scope': SCOPE,
'discoveryDocs': ['https://sheets.googleapis.com/$discovery/rest?version=v4'],
}).then(function() {
gapi.auth2.getAuthInstance().isSignedIn.listen(updateSignInStatus);
updateSignInStatus(gapi.auth2.getAuthInstance().isSignedIn.get());
});
}

function handleClientLoad() {
gapi.load('client:auth2', initClient);
}

function updateSignInStatus(isSignedIn) {
if (isSignedIn) {
//read_data();
}
}

function handleSignInClick(event) {
gapi.auth2.getAuthInstance().signIn();
}

function handleSignOutClick(event) {
gapi.auth2.getAuthInstance().signOut();
location.reload();
}