Wrong validators connected metric

[twoeths]

Describe the bug

Once a validator is connected to a beacon node, beacon node always count that validator as "connected" forever event through the validator is turned off or switched to another beacon node

Metric name is validator_monitor_validators

For the context: https://discord.com/channels/593655374469660673/743858262864167062/1072481654158938173

Expected behavior
Should not count the validator if it's disconnected from the beacon node for a while

See

lodestar/packages/beacon-node/src/metrics/validatorMonitor.ts

Line 195 in f4057cf

validators.set(index, {index, summaries: new Map<Epoch, EpochSummary>(), inSyncCommitteeUntilEpoch: -1});