Missing metric : number of Exits / Exit Rate

This issue was there :
https://github.com/googleapis/nodejs-analytics-data/issues/221
Maintainers requested that it was reposted here

**Is your feature request related to a problem? Please describe.**

There seems to be a missing metric about Exits / Exit Rate (see https://support.google.com/analytics/answer/2525491). This would be particularly useful in combination with the dimension unifiedPagePathScreen to detect the exit rate for each pages.

**Describe the solution you'd like**

Make the Exit available from Google Analytics Data API

**Describe alternatives you've considered**

Could not find a good alternative to this metric missing : we had to to check the value manually in GA 4 console.

**Additional context**

This metric exists in GA4 since it can be found from GA search bar :
https://mcusercontent.com/bfa1c8f3bfff111e8cefeeb14/images/7efc0815-fe6b-2416-df5a-13f452b4da87.jpg

Or from custom views :
https://mcusercontent.com/bfa1c8f3bfff111e8cefeeb14/images/35b73b21-8d39-685c-1008-2dbe88b0ec77.jpg