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/overseer/src/lib.rs

@@ -403,7 +403,7 @@ pub struct Overseer<SupportsParachains> {
 	#[subsystem(no_dispatch, ApprovalVotingMessage)]
 	approval_voting: ApprovalVoting,
 
-	#[subsystem(no_dispatch, GossipSupportMessage)]
+	#[subsystem(GossipSupportMessage)]
 	gossip_support: GossipSupport,
 
 	#[subsystem(no_dispatch, DisputeCoordinatorMessage)]