Replace '.' in counter names with ':'

The '.' is illegal character in counter names, as they are exported
via CTRL interface, where '.' has a special meaning that cannot be
used by strings comprising the variable name.

Change-Id: I4eb6851d3577f5942ae1a9ab28d3b3ca7ab8e208
diff --git a/src/gprs/gb_proxy_peer.c b/src/gprs/gb_proxy_peer.c
index 69cce3a..f2cdd93 100644
--- a/src/gprs/gb_proxy_peer.c
+++ b/src/gprs/gb_proxy_peer.c
@@ -73,7 +73,7 @@
 osmo_static_assert(ARRAY_SIZE(peer_ctr_description) == GBPROX_PEER_CTR_LAST, everything_described);
 
 static const struct rate_ctr_group_desc peer_ctrg_desc = {
-	.group_name_prefix = "gbproxy.peer",
+	.group_name_prefix = "gbproxy:peer",
 	.group_description = "GBProxy Peer Statistics",
 	.num_ctr = ARRAY_SIZE(peer_ctr_description),
 	.ctr_desc = peer_ctr_description,