MINOR: applet: Change return value for .init callback function

0 is now returned on success and -1 on error.
diff --git a/addons/promex/service-prometheus.c b/addons/promex/service-prometheus.c
index 8422e54..9501b1c 100644
--- a/addons/promex/service-prometheus.c
+++ b/addons/promex/service-prometheus.c
@@ -1497,7 +1497,7 @@
 {
 	applet_reserve_svcctx(appctx, sizeof(struct promex_ctx));
 	appctx->st0 = PROMEX_ST_INIT;
-	return 1;
+	return 0;
 }
 
 /* The main I/O handler for the promex applet. */