Further reduce the CPU overhead of networking metrics (#10875)

* Simplify `num_connected_peers`

* Track requested peer counts

* Revert "Track requested peer counts"

This reverts commit 9f1c8704353df6afc17ed7e9f4ab8d8e29466ae4.

* Remove `substrate_sub_libp2p_peerset_num_requested` metric

* Remove two unused functions that I forgot to get rid of in previous commit

substrate/client/network/src/protocol.rs

@@ -465,12 +465,6 @@ impl<B: BlockT> Protocol<B> {
 		self.behaviour.open_peers()
 	}
 
-	/// Returns the list of all the peers that the peerset currently requests us to be connected
-	/// to on the default set.
-	pub fn requested_peers(&self) -> impl Iterator<Item = &PeerId> {
-		self.behaviour.requested_peers(HARDCODED_PEERSETS_SYNC)
-	}
-
 	/// Returns the number of discovered nodes that we keep in memory.
 	pub fn num_discovered_peers(&self) -> usize {
 		self.behaviour.num_discovered_peers()
@@ -496,7 +490,7 @@ impl<B: BlockT> Protocol<B> {
 
 	/// Returns the number of peers we're connected to.
 	pub fn num_connected_peers(&self) -> usize {
-		self.peers.values().count()
+		self.peers.len()
 	}
 
 	/// Returns the number of peers we're connected to and that are being queried.