Implement lean_attestation_committee_subnet metric (#243)

## Motivation

The [leanMetrics
specification](https://github.com/leanEthereum/leanMetrics/blob/main/metrics.md)
defines `lean_attestation_committee_subnet` as a Network Metric (Gauge,
set on node start) that all lean clients should expose. ethlambda
already computes the subnet ID at startup (`validator_id %
attestation_committee_count`) for gossipsub topic subscription, but does
not expose it as a Prometheus gauge.

This brings ethlambda closer to full leanMetrics compliance and helps
operators verify subnet assignments via Grafana dashboards.

## Description

Register a new `lean_attestation_committee_subnet` Prometheus `IntGauge`
in the P2P metrics module and set it at startup with the computed subnet
ID.

### Changes

- **`crates/net/p2p/src/metrics.rs`**: New
`set_attestation_committee_subnet(subnet_id: u64)` function with a
function-scoped `LazyLock<IntGauge>`, following the same pattern used by
`set_attestation_committee_count` in the blockchain metrics module.

- **`crates/net/p2p/src/lib.rs`**: Call the setter after computing
`subnet_id` (line 213), passing `subnet_id.unwrap_or(0)`. Non-validator
nodes default to subnet 0, which matches their fanout publishing
behavior.

- **`docs/metrics.md`**: Added row to the Network Metrics table
documenting the new metric as supported.

### Design decisions

- **Placed in P2P metrics** (not blockchain metrics): The subnet is a
network-layer concept — it governs gossipsub topic subscription and is
computed within the P2P module.
- **Non-validator nodes expose subnet 0**: The leanMetrics spec says "on
node start" with no validator-only qualifier. Non-validators use subnet
0 for fanout publishing, so exposing 0 is accurate.

## How to Test

```bash
make fmt   # passes
make lint  # passes
make test  # all tests pass
```

To verify at runtime: start a node and check `curl
http://localhost:5054/metrics | grep lean_attestation_committee_subnet`.

---------

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: pablodeymo

crates/net/p2p/src/lib.rs

@@ -209,12 +209,11 @@ pub fn build_swarm(
     // attestation_committee_count is validated to be >= 1 by clap at CLI parse time.
     let subnet_id = config
         .validator_id
-        .map(|vid| vid % config.attestation_committee_count);
-    let attestation_topic_kind = match subnet_id {
-        Some(id) => format!("{ATTESTATION_SUBNET_TOPIC_PREFIX}_{id}"),
-        // Non-validators use subnet 0 for publishing
-        None => format!("{ATTESTATION_SUBNET_TOPIC_PREFIX}_0"),
-    };
+        .map(|vid| vid % config.attestation_committee_count)
+        .unwrap_or(0);
+    metrics::set_attestation_committee_subnet(subnet_id);
+
+    let attestation_topic_kind = format!("{ATTESTATION_SUBNET_TOPIC_PREFIX}_{subnet_id}");
     let attestation_topic_str =
         format!("/leanconsensus/{network}/{attestation_topic_kind}/ssz_snappy");
     let attestation_topic = libp2p::gossipsub::IdentTopic::new(attestation_topic_str);

crates/net/p2p/src/metrics.rs

@@ -68,6 +68,18 @@ static LEAN_PEER_DISCONNECTION_EVENTS_TOTAL: LazyLock<IntCounterVec> = LazyLock:
     .unwrap()
 });
 
+/// Set the attestation committee subnet gauge.
+pub fn set_attestation_committee_subnet(subnet_id: u64) {
+    static LEAN_ATTESTATION_COMMITTEE_SUBNET: LazyLock<IntGauge> = LazyLock::new(|| {
+        register_int_gauge!(
+            "lean_attestation_committee_subnet",
+            "Node's attestation committee subnet"
+        )
+        .unwrap()
+    });
+    LEAN_ATTESTATION_COMMITTEE_SUBNET.set(subnet_id.try_into().unwrap_or_default());
+}
+
 /// Notify that a peer connection event occurred.
 ///
 /// If `result` is "success", the connected peer count is incremented.

docs/metrics.md

@@ -74,6 +74,7 @@ The exposed metrics follow [the leanMetrics specification](https://github.com/le
 | Name   | Type  | Usage | Sample collection event | Labels | Supported |
 |--------|-------|-------|-------------------------|--------|-----------|
 |`lean_attestation_committee_count`| Gauge | Number of attestation committees | On node start | | ✅ |
+|`lean_attestation_committee_subnet`| Gauge | Node's attestation committee subnet | On node start | | ✅ |
 |`lean_connected_peers`| Gauge | Number of connected peers | On scrape | client=ethlambda,grandine,lantern,lighthouse,qlean,ream,zeam | ✅(*) |
 |`lean_peer_connection_events_total`| Counter | Total number of peer connection events | On peer connection | direction=inbound,outbound<br>result=success,timeout,error | ✅ |
 |`lean_peer_disconnection_events_total`| Counter | Total number of peer disconnection events | On peer disconnection | direction=inbound,outbound<br>reason=timeout,remote_close,local_close,error | ✅ |