Add lots of networking metrics for Prometheus (#5126)

* Add some metrics

* Address concerns

substrate/client/peerset/src/lib.rs

@@ -518,6 +518,11 @@ impl Peerset {
 		})
 	}
 
+	/// Returns the number of peers that we have discovered.
+	pub fn num_discovered_peers(&self) -> usize {
+		self.data.peers().len()
+	}
+
 	/// Returns priority group by id.
 	pub fn get_priority_group(&self, group_id: &str) -> Option<HashSet<PeerId>> {
 		self.data.get_priority_group(group_id)

substrate/client/peerset/src/peersstate.rs

@@ -144,7 +144,7 @@ impl PeersState {
 	/// Returns the list of all the peers we know of.
 	// Note: this method could theoretically return a `Peer`, but implementing that
 	// isn't simple.
-	pub fn peers(&self) -> impl Iterator<Item = &PeerId> {
+	pub fn peers(&self) -> impl ExactSizeIterator<Item = &PeerId> {
 		self.nodes.keys()
 	}