client/authority-discovery: Reduce log level replaced by metrics

Instead of logging value-found-event-handling failures or value-put
failures on error level, log them on debug level only additionally
recording them via Prometheus.

Motivation is that both events can happen in "normal" operations and
thus clutter the logs.

substrate/client/authority-discovery/src/lib.rs

@@ -60,7 +60,7 @@ use futures_timer::Delay;
 
 use codec::{Decode, Encode};
 use error::{Error, Result};
-use log::{debug, error, log_enabled, warn};
+use log::{debug, error, log_enabled};
 use prometheus_endpoint::{Counter, CounterVec, Gauge, Opts, U64, register};
 use prost::Message;
 use sc_client_api::blockchain::HeaderBackend;
@@ -328,7 +328,11 @@ where
 					}
 
 					if let Err(e) = self.handle_dht_value_found_event(v) {
-						error!(
+						if let Some(metrics) = &self.metrics {
+							metrics.handle_value_found_event_failure.inc();
+						}
+
+						debug!(
 							target: LOG_TARGET,
 							"Failed to handle Dht value found event: {:?}", e,
 						);
@@ -359,7 +363,7 @@ where
 						metrics.dht_event_received.with_label_values(&["value_put_failed"]).inc();
 					}
 
-					warn!(
+					debug!(
 						target: LOG_TARGET,
 						"Failed to put hash '{:?}' on Dht.", hash
 					)
@@ -603,6 +607,7 @@ pub(crate) struct Metrics {
 	amount_last_published: Gauge<U64>,
 	request: Counter<U64>,
 	dht_event_received: CounterVec<U64>,
+	handle_value_found_event_failure: Counter<U64>,
 	priority_group_size: Gauge<U64>,
 }
 
@@ -642,6 +647,13 @@ impl Metrics {
 				)?,
 				registry,
 			)?,
+			handle_value_found_event_failure: register(
+				Counter::new(
+					"authority_discovery_handle_value_found_event_failure",
+					"Number of times handling a dht value found event failed."
+				)?,
+				registry,
+			)?,
 			priority_group_size: register(
 				Gauge::new(
 					"authority_discovery_priority_group_size",