Add the metric <prefix>.transaction.lowest_available_committed_tx_id (neo4j#2222)

Related to neo-technology/neo4j#30053
The updated description is covered in
neo-technology/neo4j#30557

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 in the transaction logs. Track this for each instance. (Cluster) Track this for each primary and secondary. The data may be organized into separate charts, displaying a single line that increases over time. The difference between this ID and that of the `last_committed_tx_id` metric indicates the size of the window of available transactions to serve to other cluster members or backups. (counter)
 |===
 
 [[db-index-metrics]]