Hello
I have not used this myself but there seem to be metrics for it, you can also use AMBA to deploy them. Down below are some links with explanation for them, amba also provides recommended thresholds from Microsoft for alerting on these metrics.
https://learn.microsoft.com/en-us/azure/nat-gateway/nat-metrics#total-snat-connection-count
AMBA
Hope this is helpful and remember shared knowledge is the best knowledge 😊
Best Regards,
Timmy Malmgren
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