Alert when running out of NAT ports

After rolling out the changes described inhttps://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5071, machines will usually not have public IPs and will access the internet via Google Cloud NAT.

To determine how many IPs we'll need, we can use the following formula

nat_ip_count = M * P / 64,512

Where:

M = number of machines in the region (multiply by some generous number to account for future growth) P = Minimum NAT ports per VM (defaults to 64)

There are 64512 TCP and UDP (each) ports available per NAT IP. (https://cloud.google.com/nat/docs/overview#number_of_nat_ports_and_connections for context)

Introducing Cloud NAT will have created another resource that can be saturated. Assuming constant min NAT ports per VM, we risk running out of NAT ports as the number of machines per router grows. We should alert on this so that SREs can provision extra NAT IPs in advance.