Support shutdownGracePeriodSeconds in node_kubelet_config

[shlomitubul]

Add shutdown_grace_period_seconds and shutdown_grace_period_critical_pods_seconds fields to the kubelet_config block on google_container_node_pool and google_container_cluster resources.

These two fields correspond to the shutdownGracePeriodSeconds and shutdownGracePeriodCriticalPodsSeconds properties that are already exposed in the NodeKubeletConfig struct of the container v1 and v1beta1 Go API SDK, but are not currently surfaced in the Terraform provider schema.

The fields control how long a GKE node delays shutting down to gracefully terminate Pods. Per the GKE node system configuration docs:

• shutdown_grace_period_seconds: total duration the node delays shutdown. Only valid for Spot VMs. Allowed values: 0, 30, 120. Default 30.
• shutdown_grace_period_critical_pods_seconds: portion reserved for critical pods (0 to 120, must be less than shutdown_grace_period_seconds).

Reference: GCP doc states gcloud equivalent is --system-config-from-file, but it is also a first-class field on NodeKubeletConfig, so it is consistent with the rest of the kubelet_config block to surface in TF.

Why this is needed

Validation

• shutdown_grace_period_seconds is validated with validation.IntInSlice([]int{0, 30, 120}) to match the GCP-imposed allowed values.
• shutdown_grace_period_critical_pods_seconds is validated with validation.IntBetween(0, 120).

What I touched

• Schema: added 2 fields to schemaNodeConfig() kubelet_config block.
• expandKubeletConfig: read both fields and set on kConfig.
• flattenKubeletConfig: output both fields.
• Test: extended testAccContainerNodePool_withKubeletConfig with the new fields + 2 new TestCheckResourceAttr assertions.
• Docs: added entries under kubelet_config in container_cluster.html.markdown (which container_node_pool docs link to).

Out of scope (intentional)

• schemaNodePoolAutoConfigNodeKubeletConfig (the auto-config CC subset) is intentionally not extended in this PR. That schema is explicitly a subset; extending it can be a follow-up.

Release Note Template for Downstream PRs (will be copied)

container: added `node_config.kubelet_config.shutdown_grace_period_seconds` and `node_config.kubelet_config.shutdown_grace_period_critical_pods_seconds` fields to `google_container_node_pool` and `google_container_cluster` resources

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[github-actions]

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