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.