Jesse Bai Thanks for reaching here! SNAT Port Usage by the instances are not currently available as a metric, hence it would not be possible to configure an alert for SNAT Port Exhaustion.
Its recommend to follow the best practices to ensure that SNAT Port Usage is within the safe limits.
For more detailed information regarding SNAT in App service, please refer to the following article: SNAT with App Service | 4lowTheRabbit.github.io
Let us know if further query or issue remains.