Supported resource types @ https://cloud.google.com/monitoring/api/resources
MonitoredResource.newBuilder currently accepts a free string. Logging backend validates against supported resource types and does not log if it is not a supported log type.

We should have a cleaner way for the user to provide the resource type : be inferred from environment (GCE vs GAE ?) or have a custom resource type be used by default when user logs via the API.