Examples: Add example on how to setup Cloud Monitoring alerts for volume quota

[okrause]

In Nov 2021, CVS will switch to hard quotas. Instead of showing the client a 100 TiB volume, no matter how much space was provisioned, CVS will start to report the provisioned (= allocated) size.

That means, that Out-Of-Space conditions can occur much easier. Users need to monitor the fill grade of their volumes better.

Ask: As a courtesy, add an example on how to use Cloud Monitoring (StackDriver) to create alerts (https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/monitoring_alert_policy) for volume running out of space.

I have most of the required stuff in place for doing it via UI and want to provide the TF example in a few days.

[okrause]

Here is example code on how to setup CVS usage alerts. Since it is using the google provider only, we will not make it part of this repository.