One Instance-One Request at a time App Engine Flexible

I am using

• App Engine Flexible, custom runtime.
• nodejs, as base Image.
• express
• Cloud Tasks for queuing the requests
• puppeteer job

My Requirements

• 20GB RAM
• long-running process

because of my unique requirement, I want 1 request to be handled by only 1 instance. when it gets free or the request gets timed-out, only then it should get a new request. I have managed to reject other requests while the instance is processing 1 request, but not able to figure out the appropriate automatic scaling settings.

Please suggest the best way to achieve this. Thanks in advance!

from One Instance-One Request at a time App Engine Flexible