metrics: common: Add a warning function
metrics: common: Add a warning function

Sometimes we want to warn rather than die. Add a function
so we have a uniform (possibly detectable in the tools
and logs?) way of issuing a warning.

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

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