metrics: density: Add documentation
metrics: density: Add documentation

This commit includes information about the
type of metrics tests included in this section,
furthermore it adds more details about metrics test.

Signed-off-by: Arevalo, Mario Alfredo C <962470f5d16f97233f38b7e7baf23f990a7fbec3@intel.com>

MarioCarrilloA/tests,clearcontainers/tests,devimc/tests,chavafg/tests,chavafg/tests,MarioCarrilloA/tests,devimc/tests,clearcontainers/tests