Fix proposer factory prometheus registration (#1862)

The proposer wasn't registered in prometheus and thus, we could not see
its metrics.

polkadot/node/service/src/lib.rs

@@ -460,6 +460,7 @@ pub fn new_full<RuntimeApi, Executor>(
 			client.clone(),
 			transaction_pool,
 			overseer_handler.clone(),
+			prometheus_registry.as_ref(),
 		);
 
 		let babe_config = babe::BabeParams {