Add lots of networking metrics for Prometheus (#5126)

* Add some metrics

* Address concerns

substrate/client/network/src/discovery.rs

@@ -176,6 +176,11 @@ impl DiscoveryBehaviour {
 	pub fn put_value(&mut self, key: record::Key, value: Vec<u8>) {
 		self.kademlia.put_record(Record::new(key, value), Quorum::All);
 	}
+
+	/// Returns the number of nodes that are in the Kademlia k-buckets.
+	pub fn num_kbuckets_entries(&mut self) -> usize {
+		self.kademlia.kbuckets_entries().count()
+	}
 }
 
 /// Event generated by the `DiscoveryBehaviour`.
@@ -203,6 +208,9 @@ pub enum DiscoveryOut {
 
 	/// Inserting a value into the DHT failed.
 	ValuePutFailed(record::Key),
+
+	/// Started a random Kademlia query.
+	RandomKademliaStarted,
 }
 
 impl NetworkBehaviour for DiscoveryBehaviour {
@@ -330,25 +338,33 @@ impl NetworkBehaviour for DiscoveryBehaviour {
 
 		// Poll the stream that fires when we need to start a random Kademlia query.
 		while let Poll::Ready(_) = self.next_kad_random_query.poll_unpin(cx) {
-			if self.num_connections < self.discovery_only_if_under_num {
+			let actually_started = if self.num_connections < self.discovery_only_if_under_num {
 				let random_peer_id = PeerId::random();
 				debug!(target: "sub-libp2p", "Libp2p <= Starting random Kademlia request for \
 					{:?}", random_peer_id);
 
 				self.kademlia.get_closest_peers(random_peer_id);
+				true
+
 			} else {
 				debug!(
 					target: "sub-libp2p",
 					"Kademlia paused due to high number of connections ({})",
 					self.num_connections
 				);
-			}
+				false
+			};
 
 			// Schedule the next random query with exponentially increasing delay,
 			// capped at 60 seconds.
 			self.next_kad_random_query = Delay::new(self.duration_to_next_kad);
 			self.duration_to_next_kad = cmp::min(self.duration_to_next_kad * 2,
 				Duration::from_secs(60));
+
+			if actually_started {
+				let ev = DiscoveryOut::RandomKademliaStarted;
+				return Poll::Ready(NetworkBehaviourAction::GenerateEvent(ev));
+			}
 		}
 
 		// Poll Kademlia.