Environment variables are not documented

There should be a proper document describing environment variables affecting XPUM working, and what possible values they could have.

For example, `XPUM_METRICS` environment variable values needed to avoid XPUM filling disk with log spam, seem to be listed only in its header file:
https://github.com/intel/xpumanager/blob/master/core/include/xpum_api.h

Additionally, any other document mentioning these environment variables, should link the environment variable document. For example the references to `XPUM_METRICS` in XPUM DockerHub page: https://hub.docker.com/r/intel/xpumanager