Document missing metrics for blocks count in pending IBR

  @Metric("Count of blocks in pending IBR")
  private MutableGaugeLong blocksInPendingIBR;
  @Metric("Count of blocks at receiving status in pending IBR")
  private MutableGaugeLong blocksReceivingInPendingIBR;
  @Metric("Count of blocks at received status in pending IBR")
  private MutableGaugeLong blocksReceivedInPendingIBR;