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/Cargo.lock

@@ -6063,7 +6063,10 @@ name = "polkadot-gossip-support"
 version = "0.9.9"
 dependencies = [
  "assert_matches",
+ "async-trait",
  "futures 0.3.17",
+ "futures-timer 3.0.2",
+ "lazy_static",
  "polkadot-node-network-protocol",
  "polkadot-node-subsystem",
  "polkadot-node-subsystem-test-helpers",
@@ -6071,11 +6074,13 @@ dependencies = [
  "polkadot-primitives",
  "rand 0.8.4",
  "rand_chacha 0.3.1",
+ "sc-network",
  "sp-application-crypto",
  "sp-consensus-babe",
  "sp-core",
  "sp-keyring",
  "sp-keystore",
+ "sp-tracing",
  "tracing",
 ]