Sample Request - Retry Policy Sample

Hi,
I read the doc [Retrying Asynchronous Functions](https://firebase.google.com/docs/functions/retries#how_to_enable_retries), that is pretty straight forward, but I had some hard time getting it working well...

So I think it will be nice to have a sample with best practices to avoid infinite retry loops.

Maybe it's not necessary to create a new sample from scratch, just add retry Policy on an existing sample demonstrating how to retry the function and how to stop it with the good condition depending of the use case.

You're doing great job, keep going! 👍