Add a MVCC related metric

Author: NataliaIvakina

modules/ROOT/pages/monitoring/metrics/reference.adoc

@@ -368,7 +368,8 @@ label:deprecated[Deprecated in 5.15]
 |<prefix>.transaction.last_closed_tx_id|The ID of the last closed transaction. (counter)
 |<prefix>.transaction.tx_size_heap|The transactions' size on heap in bytes. (histogram)
 |<prefix>.transaction.tx_size_native|The transactions' size in native memory in bytes. (histogram)
-|<prefix>.transaction.validation_failure|The total number of multi version transaction validation failures. (counter)
+|<prefix>.transaction.validation_failure|The total number of multi-version transaction validation failures. (counter)
+|<prefix>.transaction.mvcc_retries|The total number of multi-version server internal retries. (counter)
 |===
 
 [[db-index-metrics]]