This trend metric is written in CQLinq, C# LINQ over the NDepend code model.
It ships with every new NDepend project: you can read it, adapt it, or use it as a starting point for your own custom rules.
// <TrendMetric Name="# Third-Party Methods Used" Unit="Methods" />
from m in ThirdParty.Methods
select new { m, m.MethodsCallingMe }