message: [storage] add metric "lastest_transaction_version"
  On our overview dashboard we use the commited_txns counter to track num of txns in total, which is not guaranteed to be precise (when for example prometheus clears data).
  Instead, adding a prometheus gause to reflect the latest version directly.
diff: @@ -337,14 +337,15 @@ impl LibraDB {
  .state_root_hash()
  };
  + let last_version = first_version + num_txns - 1;
  if let Some(x) = ledger_info_with_sigs {
  - let last_version = x.ledger_info().version();
  + let claimed_last_version = x.ledger_info().version();
  ensure!(
  - first_version + num_txns - 1 == last_version,
  + claimed_last_version == last_version,
  "Transaction batch not applicable: first_version {}, num_txns {}, last_version {}",
  first_version,
  num_txns,
  - last_version
  + claimed_last_version,
  );
  }
  @@ -375,6 +376,7 @@ impl LibraDB {
  self.commit(batch)?;
  // Only increment counter if commit(batch) succeeds.
  OP_COUNTER.inc_by("committed_txns", txns_to_commit.len());
  + OP_COUNTER.set("latest_transaction_version", last_version as usize);
  Ok(())
  }
  