set validator_monitor_block_miss_total metric to zero at start

[yakimant]

Describe the bug

validator_monitor_block_miss_total is missing until missed block event happens:

$ curl -s 0:8008/metrics | grep validator_monitor_block_miss
# HELP validator_monitor_block_miss Number of times the validator was expected to propose a block but no block was included
# TYPE validator_monitor_block_miss counter

It makes graphs and alerts relying on increase, rate tricky.

As a best practice it's recommended to set metrics at startup with zero.
(see links at the bottom).

To Reproduce
Steps to reproduce the behavior:

1. Run Nimbus Beacon Node with Validator Monitor
2. Check the metrics endpoint output for validator_monitor_block_miss_total

Links

• https://prometheus.io/docs/practices/instrumentation/#avoid-missing-metrics
• https://prometheus.io/docs/concepts/metric_types/#counter
• https://github.com/status-im/nimbus-eth2/blob/stable/grafana/beacon_nodes_Grafana_dashboard.json