Log info about low connectivity and unreachable validators (#3916)

* Attempt to add log stats to gossip-support.

* WIP: Keep track of connected validators.

* Clarify metric.

* WIP: Make gossip support report connectivity.

* WIP: Fixing tests.

* Fix network bridge + integrate in overseer.

* Consistent naming.

* Fix logic error

* cargo fmt

* Pretty logs.

* cargo fmt

* Use `Delay` to trigger periodic checks.

* fmt

* Fix warning for authority set size of 1.

* More correct ratio report if there are no resolved validators.

* Prettier rendering of empty set.

* Fix typo.

* Another typo.

* Don't check on every leaf update.

* Make compatible with older rustc.

* Fix tests.

* Demote warning.

polkadot/node/service/src/overseer.rs

@@ -147,7 +147,7 @@ pub fn create_default_subsystems<'a, Spawner, RuntimeClient>(
 		CollatorProtocolSubsystem,
 		ApprovalDistributionSubsystem,
 		ApprovalVotingSubsystem,
-		GossipSupportSubsystem,
+		GossipSupportSubsystem<AuthorityDiscoveryService>,
 		DisputeCoordinatorSubsystem,
 		DisputeParticipationSubsystem,
 		DisputeDistributionSubsystem<AuthorityDiscoveryService>,
@@ -236,7 +236,10 @@ where
 			Box::new(network_service.clone()),
 			Metrics::register(registry)?,
 		),
-		gossip_support: GossipSupportSubsystem::new(keystore.clone()),
+		gossip_support: GossipSupportSubsystem::new(
+			keystore.clone(),
+			authority_discovery_service.clone(),
+		),
 		dispute_coordinator: DisputeCoordinatorSubsystem::new(
 			parachains_db.clone(),
 			dispute_coordinator_config,