The Stackdriver Basic tier, a free tier of service that provides basic
features, a moderate allotment of logs, and access to built-in metrics.
A number of features are not available in this tier. For more details,
see the service tiers
The Stackdriver Premium tier, a higher, more expensive tier of service
that provides access to all Stackdriver features, lets you use Stackdriver
with AWS accounts, and has a larger allotments for logs and metrics. For
more details, see the service tiers
An invalid sentinel value, used to indicate that a tier has not
been provided explicitly.