compute: support maxPortsPerVm field related to Cloud NAT's enableDynamicPortAllocation

[dark]

Support for the Dynamic Port Allocation feature (tracked in
terraform-google-modules/terraform-google-cloud-nat#64 and
hashicorp/terraform-provider-google#11052) was initially implemented
in #6022, but it lacked support for the maxPortsPerVm field. This
field is crucial to allow the full configuration to work.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: add maxPortsPerVm field to `google_compute_router_nat` resource

[modular-magician]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 108 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 3 files changed, 108 insertions(+), 2 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[dark]

I think this might be a duplicate of #6127?

You are right, I missed that PR.
But this one has a test and a more descriptive comment :)

[melinath]

It looks like this probably supersedes #6127 since it already has tests and docs updates - attn @prateek2408

[modular-magician]

Tests analytics

Total tests: 2053
Passed tests 1824
Skipped tests: 226
Failed tests: 3

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccContainerCluster_withConfidentialNodes|TestAccContainerCluster_withAddons|TestAccComputeRouterNat_withPortAllocationMethods

[modular-magician]

Tests passed during RECORDING mode:
TestAccComputeRouterNat_withPortAllocationMethods[view]

Tests failed during RECORDING mode:
TestAccContainerCluster_withConfidentialNodes[view]
TestAccContainerCluster_withAddons[view]

Please fix these to complete your PR
View the build log or the debug log for each test

[melinath]

LGTM but one question

[melinath]

If this field isn't set, does the API return a default of 65536, or does it see the field as NULL (in some way) but this is the server-side behavior if the field is NULL?

If the API returns a default, then this field (and min_ports_per_vm) should be marked with default_from_api in a terraform.yaml file. Example:

magic-modules/mmv1/products/networkservices/terraform.yaml

Line 51 in c4f5dc1

protocol: !ruby/object:Overrides::Terraform::PropertyOverride

[dark]

The second alternative: when the field is not set, a server-side of 65536 is used, but the resource still contains a null.