C#: Add telemetry query for supported sources.

csharp/ql/src/Telemetry/SupportedExternalSources.ql

@@ -0,0 +1,17 @@
+/**
+ * @name Supported sources in external libraries
+ * @description A list of 3rd party APIs detected as sources. Excludes test and generated code.
+ * @kind metric
+ * @tags summary
+ * @id csharp/telemetry/supported-external-api-sources
+ */
+
+import csharp
+import ExternalAPI
+
+from ExternalAPI api, int usages
+where
+  not api.isUninteresting() and
+  api.isSource() and
+  usages = strictcount(Call c | c.getTarget() = api)
+select api.getInfo() as info, usages order by usages desc