Praji,
The table incident_metric is a database view that joins together the Metric Definition, Metric Instance and Incident tables. Given that your concern appears to be around assignment group changes, I think the first filter you will want is against the Metric Defintion Name field. That field should just show up as 'Name' in your field list (as the other tables don't have a Name field). You'd want to filter that down to Name = 'Assignment Group'. This would at least get you to the right metric (I believe). The concept of determining 'reassignment' back to the Service Desk might be a bit trickier. Metrics just keep track of field changes (at their basic level). You could filter down the field name Value being the name of your Service Desk group...but that would also get you times it was assigned to that group initially too. A better approach might be to actually create a scripted metric that makes the entries you need based on a concept of reassignment and that way you JUST get what you want logged.
