Get an EngineSampler assuming some sensible defaults.

This uses the environment variable GOOGLE_CLOUD_PROJECT for the Engine project_id, unless set explicitly.

processor_id Engine processor ID (from Cloud console or Engine.list_processors).
project_id Optional explicit Google Cloud project id. Otherwise, this defaults to the environment variable GOOGLE_CLOUD_PROJECT. By using an environment variable, you can avoid hard-coding personal project IDs in shared code.

EnvironmentError If no project_id is specified and the environment variable GOOGLE_CLOUD_PROJECT is not set.