Emit cpu_util from transformer instead of pollster

The emission of the cpu_util counter from the compute CPUUtilPollster
can be superceeded by the RateOfChangeTransformer (the intended
target usercase for the new transformer).

The caching of the CPUStats is now no longer required.

Note that the pipeline.yaml must be updated in deployments, otherwise
the cpu_util meter will no longer be collected.

Change-Id: I79ff047595b960de7a2acb2ec81c357fe2ee21e5
Merge "Emit cpu_util from transformer instead of pollster"