add some more tracing

Signed-off-by: Iulian Barbu <iulian.barbu@parity.io>

cumulus/client/relay-chain-rpc-interface/src/rpc_client.rs

@@ -149,7 +149,10 @@ impl RelayChainRpcClient {
 			metrics: prometheus_registry
 				.and_then(|inner| BlockchainRpcMetrics::register(inner).map_err(|err| {
 					tracing::warn!(target:LOG_TARGET, error = %err, "Unable to instantiate the RPC client metrics, continuing w/o metrics setup.");
-				}).ok()),
+				}).ok().and_then(|inner| {
+					tracing::debug!("BlockchainRpcMetrics were installed successfully.");
+					Some(inner)
+				})),
 		}
 	}