Expose lean_attestation_committee_subnet as a Prometheus gauge

Register and set the metric at P2P startup with the computed subnet ID
(validator_id % attestation_committee_count), per the leanMetrics spec.
Non-validator nodes default to subnet 0.

Author: pablodeymo

crates/net/p2p/src/lib.rs

@@ -210,6 +210,8 @@ pub fn build_swarm(
     let subnet_id = config
         .validator_id
         .map(|vid| vid % config.attestation_committee_count);
+    metrics::set_attestation_committee_subnet(subnet_id.unwrap_or(0));
+
     let attestation_topic_kind = match subnet_id {
         Some(id) => format!("{ATTESTATION_SUBNET_TOPIC_PREFIX}_{id}"),
         // Non-validators use subnet 0 for publishing

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());
+}
+
 /// 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 | ✅ |