Explanation/Reference: https://cloud.google.com/compute/docs/instances/adding-removing-ssh-keys

You can enable GKE Sandbox on your cluster to isolate untrusted workloads in sandboxes on the node. GKE Sandbox is built using gVisor, an open source project.
https://cloud.google.com/kubernetes-engine/docs/concepts/security-
overview?hl=en#protecting_nodes_from_untrusted_workloads

As to mexblood1's point, CPU utilization is a recommended proxy for traffic when it comes to Cloud Spanner. See: Alerts for high CPU utilization The following table specifies our recommendations for maximum CPU usage for both single-region and multi-region instances. These numbers are to ensure that your instance has enough compute capacity to continue to serve your traffic in the event of the loss of an entire zone (for single-region instances) or an entire region (for multi-region instances). - https://cloud.google.
com/spanner/docs/cpu-utilization

Explanation
In Google compute engine, if predefined machine types don't meet your needs, you can create an instance with custom virtualized hardware settings. Specifically, you can create an instance with a custom number of vCPUs and custom memory, effectively using a custom machine type. Custom machine types are ideal for the following scenarios: 1. Workloads that aren't a good fit for the predefined machine types that are available to you. 2. Workloads that require more processing power or more memory but don't need all of the upgrades that are provided by the next machine type level.In our scenario, we only need a memory upgrade. Moving to a bigger instance would also bump up the CPU which we don't need so we have to use a custom machine type.
It is not possible to change memory while the instance is running so you need to first stop the instance, change the memory and then start it again. See below a screenshot that shows how CPU/Memory can be customized for an instance that has been stopped.Ref: https://cloud.google.com/compute/docs/instances/creating-instance-with-custom-machine-type

Coldline Storage is a very-low-cost, highly durable storage service for storing infrequently accessed data.
Coldline Storage is ideal for data you plan to read or modify at most once a quarter. Since we have a requirement to access data once a quarter and want to go with the most cost-efficient option, we should select Coldline Storage.
Ref: https://cloud.google.com/storage/docs/storage-classes#coldline