validator_monitor_slashed is showing zero until validators loaded

[yakimant]

Describe the bug
validator_monitor_slashed is dropped to zero after node resart and then recovers to previous setting.
This makes difficult to catch the slashed validators. Restart causes drop/spike.
We can workaround it, but it's not obvious for other users and causes either delayed or false positive alerts.

To Reproduce

1. Prepare a node with validator monitoring and slashed validator
2. Read the validator_monitor_slashed metric (not zero)
3. Restart the node
4. Read the validator_monitor_slashed metric (zero)
5. Give it some time
6. Read the validator_monitor_slashed metric (not zero)

Screenshots

Possible solutions

1. Don't provide the metric until all validators are loaded
2. Reset the counter on node restart completely