[ndnSIM] where the FaceMetric is used?

[Mortaza Nikzad]

Hi,
In ndnSIM, each face has an attribute named "FaceMetric". We can set or get the value of this attribute by face.setMetric() and face.getMetric() in forwarder. I cannot the usage of this attribute in current version of simulator. can anyone tell me please, where the FaceMetric is used in current version of simulator? I want to use it for developing of  my new strategy. is it safe?
thanks