Add flag for collecting tethering active sessions metrics

This is needed for follow-up CLs to wrap necessary code
change under this flag.

Test: TH
Bug: 354619988
Change-Id: I7a36ff228b1ff34860c7fa4e90537ad87f543ddc
diff --git a/common/flags.aconfig b/common/flags.aconfig
index 45cbb78..4c6d8ba 100644
--- a/common/flags.aconfig
+++ b/common/flags.aconfig
@@ -147,3 +147,12 @@
   bug: "335680025"
   is_fixed_read_only: true
 }
+
+flag {
+  name: "tethering_active_sessions_metrics"
+  is_exported: true
+  namespace: "android_core_networking"
+  description: "Flag for collecting tethering active sessions metrics"
+  bug: "354619988"
+  is_fixed_read_only: true
+}