Document metrics reporting -1 on block format. (neo4j#2311)

Cherry-picked from neo4j#2288

---------

Co-authored-by: Satia Herfert <[email protected]>

Author: renetapopova

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

@@ -229,6 +229,13 @@ label:deprecated[Deprecated in 5.15]
 |The total number of internally generated reusable IDs for the nodes stored in the database. These IDs do not reflect changes in the actual data. If you want to have a rough metric of how big your graph is, use `<prefix>.neo4j.count.node` instead. (gauge)
 |===
 
+[NOTE]
+====
+Database data metrics do not report values for databases using xref:database-internals/store-formats.adoc[block format].
+Instead, the metrics always report `-1`.
+This is not an indication of an error of any kind.
+====
+
 [[global-neo4j-pools-metrics]]
 === Global neo4j pools metrics