Wrong validators connected metric

**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 https://github.com/ChainSafe/lodestar/blob/f4057cfc3f74e62cdf7452dd4215b482cffd756d/packages/beacon-node/src/metrics/validatorMonitor.ts#L195