Add the metric <prefix>.transaction.lowest_available_committed_tx_id

Author: NataliaIvakina

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

@@ -365,6 +365,7 @@ label:deprecated[Deprecated in 5.15]
 |<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.mvcc_retries|The total number of multi-version server internal retries. (counter)
+|<prefix>.transaction.lowest_available_committed_tx_id| label:new[Introduced in 2025.04] The ID of the lowest available committed transaction present in the transaction logs. Track this for each instance. (Cluster) Track this for each primary, and each secondary. Might break into separate charts. It should show one line, ever increasing. The difference between this ID and that of `last_committed_tx_id` metric indicates the size of the window of available transactions to serve to other cluster members or backup (counter)
 |===
 
 [[db-index-metrics]]