message: [network] fix connected peers counter
  Closes:
  Approved by: davidiw
diff: @@ -357,10 +357,10 @@ where
  ConnectionNotification::NewConnection(conn) => {
  info!("New connection established: {:?}", conn,);
  // Update libra_network_peer counter.
  + self.add_peer(conn);
  counters::LIBRA_NETWORK_PEERS
  .with_label_values(&[self.role.as_str(), "connected"])
  - .inc();
  - self.add_peer(conn);
  + .set(self.active_peers.len() as i64);
  }
  ConnectionNotification::Disconnected(lost_conn_metadata, reason) => {
  // See: https://github.com/libra/libra/issues/3128#issuecomment-605351504 for
  @@ -369,10 +369,6 @@ where
  "Connection {:?} closed due to {:?}",
  lost_conn_metadata, reason,
  );
  - // Update libra_network_peer counter.
  - counters::LIBRA_NETWORK_PEERS
  - .with_label_values(&[self.role.as_str(), "connected"])
  - .dec();
  let peer_id = lost_conn_metadata.peer_id();
  // If the active connection with the peer is lost, remove it from `active_peers`.
  if let Entry::Occupied(entry) = self.active_peers.entry(peer_id) {
  @@ -382,6 +378,9 @@ where
  entry.remove();
  }
  }
  + counters::LIBRA_NETWORK_PEERS
  + .with_label_values(&[self.role.as_str(), "connected"])
  + .set(self.active_peers.len() as i64);
  // If the connection was explicitly closed by an upstream client, send an ACK.
  if let Some(oneshot_tx) = self
  