Add polkadot_parachain_peer_connectivity metric (#8973)

# Description

Fixes #8911

Adds `polkadot_parachain_peer_connectivity` histogram metric to better
understand connectivity patterns.

## Integration

Doesn't affect downstream projects.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: AndreiEres

polkadot/node/network/bridge/src/metrics.rs

@@ -47,12 +47,11 @@ impl Metrics {
 	}
 
 	pub fn note_peer_count(&self, peer_set: PeerSet, version: ProtocolVersion, count: usize) {
-		self.0.as_ref().map(|metrics| {
-			metrics
-				.peer_count
-				.with_label_values(&[peer_set_label(peer_set, version)])
-				.set(count as u64)
-		});
+		if let Some(metrics) = self.0.as_ref() {
+			let label = peer_set_label(peer_set, version);
+			metrics.peer_count.with_label_values(&[label]).set(count as u64);
+			metrics.peer_connectivity.with_label_values(&[label]).observe(count as f64);
+		}
 	}
 
 	pub fn on_notification_received(
@@ -131,6 +130,7 @@ impl Metrics {
 #[derive(Clone)]
 pub(crate) struct MetricsInner {
 	peer_count: prometheus::GaugeVec<prometheus::U64>,
+	peer_connectivity: prometheus::HistogramVec,
 	connected_events: prometheus::CounterVec<prometheus::U64>,
 	disconnected_events: prometheus::CounterVec<prometheus::U64>,
 	desired_peer_count: prometheus::GaugeVec<prometheus::U64>,
@@ -165,6 +165,16 @@ impl metrics::Metrics for Metrics {
 				)?,
 				registry,
 			)?,
+			peer_connectivity: prometheus::register(
+				prometheus::HistogramVec::new(
+					prometheus::HistogramOpts::new(
+						"polkadot_parachain_peer_connectivity",
+						"Histogram of peer counts on a parachain-related peer-set to track connectivity patterns",
+					).buckets(vec![0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0, 40.0, 50.0, 100.0, 250.0, 500.0, 1000.0]),
+					&["protocol"]
+				)?,
+				registry,
+			)?,
 			connected_events: prometheus::register(
 				prometheus::CounterVec::new(
 					prometheus::Opts::new(

prdoc/pr_8973.prdoc

@@ -0,0 +1,9 @@
+title: Add polkadot_parachain_peer_connectivity metric
+doc:
+- audience: Node Dev
+  description: |-
+    Adds `polkadot_parachain_peer_connectivity` histogram metric to better understand connectivity patterns.
+
+crates:
+- name: polkadot-network-bridge
+  bump: patch