Metric: The identifier of the Cloud Monitoring metric. The metric cannot have negative values and should be a utilization metric, which means that the number of virtual machines handling requests should increase or decrease proportionally to the metric. The metric must also have a label of compute.googleapis.com/resource_id with the value of the instance's unique ID, although this alone does not guarantee that the metric is valid.
For example, the following is a valid metric: compute.googleapis.com/instance/network/received_bytes_count
The following is not a valid metric because it does not increase or decrease based on usage: compute.googleapis.com/instance/cpu/reserved_cores