cleanup: turn a warning into an error

Signed-off-by: Andrea Terzolo <andreaterzolo3@gmail.com>

userspace/falco/app/actions/process_events.cpp

@@ -432,7 +432,7 @@ static falco::app::run_result init_stats_writer(
 
 	if (config->m_metrics_enabled && !sw->has_output())
 	{
-		falco_logger::log(LOG_WARNING, "Metrics are enabled with no output configured, no snapshot will be collected");
+		return falco::app::run_result::fatal("Metrics are enabled with no output configured. Please enable at least one output channel");
 	}
 
 	falco_logger::log(LOG_INFO, "Setting metrics interval to " + config->m_metrics_interval_str + ", equivalent to " + std::to_string(config->m_metrics_interval) + " (ms)\n");