Make Excel multi-tab metric compatible with PG <= 15 (#7832)

## Rationale
The `Excel multi-tab` metric uses a subquery without an alias which
isn't supported until Postgres 16+

## Changes
- Add an alias

## Tasks 📍
- [x] Claude Code Review
-  ~Manual Testing~
- ~Test Automation~

Author: labkey-jeckels

query/src/org/labkey/query/persist/QueryManager.java

@@ -1125,7 +1125,7 @@ private static Map<String, Object> getExportTypeCountsMetric()
                         "                            AND CreatedBy IS NOT NULL\n" +
                         "                            GROUP BY CreatedBy, DATE_TRUNC('second', created)\n" +
                         "                            ) AS subquery\n" +
-                        "                      WHERE subquery.count > 1)");
+                        "                      WHERE subquery.count > 1) AS multitab");
                 counts.put("Excel multi-tab", new SqlSelector(dbSchema, sql)
                         .getObject(Long.class));
             }