Describes the cause of the error with structured details.

Example of an error when contacting the "pubsub.googleapis.com" API when it
is not enabled:

{ "reason": "API_DISABLED"
  "domain": "googleapis.com"
  "metadata": {
    "resource": "projects/123",
    "service": "pubsub.googleapis.com"
  }
}

This response indicates that the pubsub.googleapis.com API is not enabled.

Example of an error that is returned when attempting to create a Spanner
instance in a region that is out of stock:

{ "reason": "STOCKOUT"
  "domain": "spanner.googleapis.com",
  "metadata": {
    "availableRegions": "us-central1,us-east2"
  }
}