Add validator metric for missed blocks

[p2p-sergey-y]

Description

Hello! Currently, validator monitoring feature allows monitoring plenty of useful things in beacon node about validators, gathering information from api/gossip/blockchains. We also can monitor new blocks produced by our validators via validator_monitor_beacon_block_totalmetric and validator_monitor_prev_epoch_beacon_blocks_total, but we can't see missed blocks. I think it will be a very useful metric for everyone to monitor not only produced blocks but also missed blocks.