The internal name of a Metric Family can be found in the Data Aggregator Data source UI
To find the internal name of any Metric Family:
1) In CA Performance Center navigate to:
2) Mouse over the column headers under the Metric Family list.
Click the Gear that appears->Columns->Internal Name
3) Find the Metric Family in question and note the internal name.
For example, the internal name for the Interface Metric Family is: