plugin_proxy: allow metrics type for output plugin proxy

Previously, the output plugin proxy supported only logs types as this is the default if no event_type is specified. This allows the metrics type to also be routed to output proxy plugins such as the golang proxy plugin.

src/flb_plugin_proxy.c

@@ -363,6 +363,7 @@ static int flb_proxy_register_output(struct flb_plugin_proxy *proxy,
 
     /* Plugin registration */
     out->type  = FLB_OUTPUT_PLUGIN_PROXY;
+    out->event_type = FLB_OUTPUT_LOGS | FLB_OUTPUT_METRICS;
     out->proxy = proxy;
     out->flags = def->flags;
     out->name  = flb_strdup(def->name);