Add flag definition for metered network firewall chains

aosp/3020885 added metered network firewall chains API but this CL
missed to add flag definition.

Test: TH
Bug: 332628891
Change-Id: I450370c6ff15da110502f8111fa491516fecbc9a
diff --git a/common/flags.aconfig b/common/flags.aconfig
index 30931df..512677a 100644
--- a/common/flags.aconfig
+++ b/common/flags.aconfig
@@ -83,3 +83,11 @@
   description: "Flag for API to register nsd offload engine"
   bug: "301713539"
 }
+
+flag {
+  name: "metered_network_firewall_chains"
+  is_exported: true
+  namespace: "android_core_networking"
+  description: "Flag for metered network firewall chain API"
+  bug: "332628891"
+}