When logging on Google Container Engine, how do I communicate to the logging platform which level each message corresponds to?
That is, the logging platform supports associating each message with a log level, but I don't know how to do this when writing log messages from my application.
If you're using the V2 API (beta), LogSeverity should be what you're looking for. (V1 as well, possibly. Haven't checked).
Currently there's only one way to get severity from your application into Cloud Logging without swapping out the default logging agent on the node (which actually isn't that hard): configure your app to log structured JSON. JSON will be automatically parsed by the logging agent, and the "severity" field will be understood by the platform if it's present.