Add lodestar prefix to validator monitor metrics

### Problem description

Validator monitor metrics `validator_monitor_` should be also prefixed with `lodestar_`:
`lodestar_validator_monitor_` instead of `validator_monitor_`. Potentially it should be fixed in 2.0 release.

Originally posted by @nflaig in [#7722 (comment)](https://github.com/ChainSafe/lodestar/pull/7722#discussion_r2067379261)

### Solution description

Add `lodestar_` prefix for all `validator_monitor_` metrics.

### Additional context

_No response_