Eagerly register invalid counter metrics at startup (#245)

## Motivation

Metrics using function-scoped `LazyLock` statics were only registered
with Prometheus on first use. Counters that may never be incremented
(e.g. invalid attestation counters) were entirely absent from `/metrics`
output, causing:

1. **Dashboard gaps**: Panels show "No data" instead of `0`
2. **Alerting blind spots**: PromQL expressions like
`rate(lean_attestations_invalid_total[5m]) > 0` silently fail when the
metric doesn't exist

Flagged by the
[leanMetrics](https://github.com/leanEthereum/leanMetrics/) team during
devnet-3 metrics review.

## Description

Moves **all** blockchain metrics from function-scoped `LazyLock` statics
to module-scoped statics, and adds a `metrics::init()` function that
eagerly registers every metric at startup via `LazyLock::force()`.

### Changes

- **`crates/blockchain/src/metrics.rs`**: All 28 metric statics (14
gauges, 10 counters, 8 histograms) are now at module scope, organized by
type (Gauges / Counters / Histograms). The `init()` function forces all
of them. Public API functions are now thin wrappers that reference the
module-scope statics directly.
- **`bin/ethlambda/src/main.rs`**: Calls `metrics::init()` at startup
before other metric initialization.

### Why all metrics, not just the invalid counters?

Per review feedback from @MegaRedHand — initializing all metrics makes
the pattern consistent and ensures the full metric set is visible from
the first Prometheus scrape. This also aligns with the module-scope
style already used in `state_transition/src/metrics.rs` and
`net/p2p/src/metrics.rs`.

## How to Test

1. Start a node: `cargo run`
2. Query the metrics endpoint: `curl localhost:5054/metrics`
3. All metrics should appear immediately, including zero-value counters:
   ```
   lean_attestations_invalid_total 0
   lean_pq_sig_aggregated_signatures_invalid_total 0
   lean_pq_sig_attestation_signatures_invalid_total 0
   lean_fork_choice_reorgs_total 0
   ...
   ```

Author: pablodeymo

bin/ethlambda/src/main.rs

@@ -81,7 +81,8 @@ async fn main() -> eyre::Result<()> {
 
     let options = CliOptions::parse();
 
-    // Set node info metrics
+    // Initialize metrics
+    ethlambda_blockchain::metrics::init();
     ethlambda_blockchain::metrics::set_node_info("ethlambda", version::CLIENT_VERSION);
     ethlambda_blockchain::metrics::set_node_start_time();
     ethlambda_blockchain::metrics::set_attestation_committee_count(