What is the metric needed to measure when a guest migrates? 

From the vmware Metrics, it appears that one of the following could be used, but these are integer values and will most likely not work for this requirement: Num Storage VMotions, Num VMotions

UIM 8.51
vmware 6.87

Unfortunately, the Total Time Metric will not perform this functionality.
The vMotion metrics do not have much data except one metric for how many vMotions were performed in a CLUSTER. Apart from that, there is only "vMotion Storage".