Complex headers+messages Millau<->Rialto relay (#878)

* complex headers+messages relay

* post-merge fix

* fix + test issue with on-demand not starting

bridges/relays/exchange/src/exchange_loop.rs

@@ -78,6 +78,11 @@ impl<BlockNumber: Clone + Copy> TransactionProofsRelayStorage for InMemoryStorag
 	}
 }
 
+/// Return prefix that will be used by default to expose Prometheus metrics of the exchange loop.
+pub fn metrics_prefix<P: TransactionProofPipeline>() -> String {
+	format!("{}_to_{}_Exchange", P::SOURCE_NAME, P::TARGET_NAME)
+}
+
 /// Run proofs synchronization.
 pub async fn run<P: TransactionProofPipeline>(
 	storage: impl TransactionProofsRelayStorage<BlockNumber = BlockNumberOf<P>>,
@@ -89,12 +94,9 @@ pub async fn run<P: TransactionProofPipeline>(
 	let exit_signal = exit_signal.shared();
 
 	relay_utils::relay_loop(source_client, target_client)
-		.with_metrics(
-			format!("{}_to_{}_Exchange", P::SOURCE_NAME, P::TARGET_NAME),
-			metrics_params,
-		)
-		.loop_metric(ExchangeLoopMetrics::default())?
-		.standalone_metric(GlobalMetrics::default())?
+		.with_metrics(Some(metrics_prefix::<P>()), metrics_params)
+		.loop_metric(|registry, prefix| ExchangeLoopMetrics::new(registry, prefix))?
+		.standalone_metric(|registry, prefix| GlobalMetrics::new(registry, prefix))?
 		.expose()
 		.await?
 		.run(|source_client, target_client, metrics| {