MINOR: stats: pass the appctx flags to stats_fill_info()

Currently the stats filling function knows nothing about the caller's
needs, so let's pass the STAT_* flags so that it can adapt to the
requester's constraints.
diff --git a/addons/promex/service-prometheus.c b/addons/promex/service-prometheus.c
index e6023d3..ee7c38f 100644
--- a/addons/promex/service-prometheus.c
+++ b/addons/promex/service-prometheus.c
@@ -545,7 +545,7 @@
 	size_t max = htx_get_max_blksz(htx, channel_htx_recv_max(chn, htx));
 	int ret = 1;
 
-	if (!stats_fill_info(info, INF_TOTAL_FIELDS))
+	if (!stats_fill_info(info, INF_TOTAL_FIELDS, 0))
 		return -1;
 
 	for (; appctx->st2 < INF_TOTAL_FIELDS; appctx->st2++) {