message: [vm] fix execution count for user transactions
  A previous refactoring of this code changed the user transaction counter
  to also include system transactions. Fix it to only count the user
  transactions.
  Closes:
diff: @@ -584,7 +584,18 @@ impl LibraVM {
  }
  Ok(PreprocessedTransaction::UserTransaction(txn)) => {
  let _timer = TXN_TOTAL_SECONDS.start_timer();
  - self.execute_user_transaction(data_cache, &txn)
  + let (vm_status, output) = self.execute_user_transaction(data_cache, &txn);
  +
  + // Increment the counter for user transactions executed.
  + let counter_label = match output.status() {
  + TransactionStatus::Keep(_) => Some("success"),
  + TransactionStatus::Discard(_) => Some("discarded"),
  + TransactionStatus::Retry => None,
  + };
  + if let Some(label) = counter_label {
  + USER_TRANSACTIONS_EXECUTED.with_label_values(&[label]).inc();
  + }
  + (vm_status, output)
  }
  Ok(PreprocessedTransaction::WriteSet(txn)) => {
  self.process_writeset_transaction(data_cache, *txn)?
  @@ -599,16 +610,6 @@ impl LibraVM {
  should_restart = true;
  }
  - // Increment the counter for transactions executed.
  - let counter_label = match output.status() {
  - TransactionStatus::Keep(_) => Some("success"),
  - TransactionStatus::Discard(_) => Some("discarded"),
  - TransactionStatus::Retry => None,
  - };
  - if let Some(label) = counter_label {
  - USER_TRANSACTIONS_EXECUTED.with_label_values(&[label]).inc();
  - }
  -
  // `result` is initially empty, a single element is pushed per loop iteration and
  // the number of iterations is bound to the max size of `signature_verified_block`
  assume!(result.len() < usize::max_value());
  