ns2: Introduce a per-bind stat_item group with backlog length

The backlog length indicates the instantaneous length of the backlog.

Change-Id: I1c55b4619b1221d7e607ace58649323407faf86b
diff --git a/src/gb/gprs_ns2_vty.c b/src/gb/gprs_ns2_vty.c
index c2b9bd9..5d0f5c0 100644
--- a/src/gb/gprs_ns2_vty.c
+++ b/src/gb/gprs_ns2_vty.c
@@ -1374,6 +1374,10 @@
 {
 	if (bind->dump_vty)
 		bind->dump_vty(bind, vty, stats);
+
+	if (stats) {
+		vty_out_stat_item_group(vty, "  ", bind->statg);
+	}
 }
 
 static void dump_ns_bind(struct vty *vty, const struct gprs_ns2_inst *nsi, bool stats)