[statistics] Provide basic statistics for the NAT

Count number of SCCP connections, number of BSC reconnects,
number of calls. For most of them we have a per BSC and a
global count.

Right now all structs using the counters survive until the
end of the application so we do not need to free them.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 3307e68..36a64b7 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -45,6 +45,11 @@
 	INIT_LLIST_HEAD(&nat->sccp_connections);
 	INIT_LLIST_HEAD(&nat->bsc_connections);
 	INIT_LLIST_HEAD(&nat->bsc_configs);
+	nat->stats.sccp.conn = counter_alloc("nat.sccp.conn");
+	nat->stats.sccp.calls = counter_alloc("nat.sccp.calls");
+	nat->stats.bsc.reconn = counter_alloc("nat.bsc.conn");
+	nat->stats.bsc.auth_fail = counter_alloc("nat.bsc.auth_fail");
+	nat->stats.msc.reconn = counter_alloc("nat.msc.conn");
 	return nat;
 }
 
@@ -72,6 +77,10 @@
 	llist_add(&conf->entry, &nat->bsc_configs);
 	++nat->num_bsc;
 
+	conf->stats.sccp.conn = counter_alloc("nat.bsc.sccp.conn");
+	conf->stats.sccp.calls = counter_alloc("nat.bsc.sccp.calls");
+	conf->stats.net.reconn = counter_alloc("nat.bsc.net.reconnects");
+
 	return conf;
 }