message: [mempool] fix counter for pending broadcasts
  Closes:
diff: @@ -315,7 +315,7 @@ impl PeerManager {
  .observe(num_txns as f64);
  counters::SHARED_MEMPOOL_PENDING_BROADCASTS_COUNT
  .with_label_values(&[network_id, &peer_id_str])
  - .inc();
  + .set(state.broadcast_info.sent_batches.len() as i64);
  counters::SHARED_MEMPOOL_BROADCAST_LATENCY
  .with_label_values(&[network_id, &peer_id_str])
  .observe(start_time.elapsed().as_secs_f64());
  