metrics: density: add proxy and rand_name to lib
metrics: density: add proxy and rand_name to lib

In preparation for adding some memory density scripts, add the
path to the proxy, and a helper function to generate 'random names'
to the metrics common library.

Signed-off-by: Graham Whaley <95f36e5ac6982f4574a5c449c37cf652e934d2b9@intel.com>

MarioCarrilloA/tests,MarioCarrilloA/tests,clearcontainers/tests,chavafg/tests,sboeuf/tests,devimc/tests,sboeuf/tests,devimc/tests,chavafg/tests,bwarden/tests,clearcontainers/tests,bwarden/tests