Clarify behavior when metric unit is not found

[mm318]

As one can see here

cloudwatch-common/cloudwatch_metrics_common/include/cloudwatch_metrics_common/utils/metric_object.h

Line 149 in 2d8f586

datum.SetUnit(Aws::CloudWatch::Model::StandardUnitMapper::GetStandardUnitForName(unit_name));

If the user-specified unit (which is a free-form std::string) is not found in unit_mapper, it will fallback to using the AWS C++ SDK's GetStandardUnitForName() function to find the corresponding relevant StandardUnit. GetStandardUnitForName() may return StandardUnit::NOT_SET.

Is this the best behavior? Why have a unit_mapper if the AWS C++ SDK already provides a GetStandardUnitForName() function?