Metrics registration can panic at startup

## Problem

RPC metrics in `crates/rpc/src/metrics.rs` use `expect()` during lazy static initialization, which will panic if metric registration fails.

## Details

```rust
static ref RPC_REQUESTS_TOTAL: IntCounterVec = IntCounterVec::new(...)
.expect("Failed to register rpc_requests_total metric")
```

All metric registrations follow this pattern:
- `RPC_REQUESTS_TOTAL`
- `RPC_REQUEST_DURATION`
- `RPC_ACTIVE_CONNECTIONS`
- `RPC_WS_SUBSCRIPTIONS`
- `RPC_RATE_LIMIT_REJECTIONS`

## Impact

- If Prometheus registry has naming conflicts or is exhausted, node crashes at startup
- No graceful degradation to running without metrics