Is your feature request related to a problem? Please describe.

Current snippets available on the official doc only mentions very primitive use case. However, specifying log level is very common use case and that use case has tricky part (c.f. #739) to handle, so the snippets should touch it.

Describe the solution you'd like

Change log_entry snippet to assign log level explicitly.

Describe alternatives you've considered

Add comments in the document on how to achieve it.

Additional context

Users need to import extra module (google.logging.type.log_severity_pb2) just to specify log level, so the process sounds more complex that the users usually think. (eg. just passing "info" string)