Add new metric message_processing_timer

Author: RagnarW

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

@@ -464,6 +464,7 @@ This is not an indication of an error of any kind.
 |<prefix>.cluster.raft.raft_log_entry_prefetch_buffer.sync_put|Raft Log Entry Prefetch buffer sync puts. (gauge)
 |<prefix>.cluster.raft.message_processing_delay|Delay between receiving a Raft message and starting its processing.
 (gauge)
+|<prefix>.cluster.raft.message_processing_delay_timer|label:new[Introduced in 2025.07] Metrics Timer for Raft message processing delays. The Timer provides different types of statistics, for example, min, max, p999, and mean_rate. (counter, histogram)
 |<prefix>.cluster.raft.message_processing_timer|Metrics Timer for Raft message processing, independent of the message type. The Timer provides different types of statistics, for example, `min`, `max`, `p999`, and `mean_rate`. (counter, histogram)
 |<prefix>.cluster.raft.replication_new|The total number of Raft replication requests. It increases with write transactions (possibly internal) activity. (counter)
 |<prefix>.cluster.raft.replication_attempt|The total number of Raft replication requests attempts. It is bigger or equal to the replication requests. (counter)