validator_monitor_slashed is showing zero until validators loaded
**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**
<img width="1428" alt="Image" src="https://github.com/user-attachments/assets/3fe6a12d-4e7f-44d8-90d7-c871f2255af3" />

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