Export db size metrics for 10 user databases (#7857)

## Problem

One database is too limiting. We have agreed to raise this limit to 10.

## Checklist before requesting a review

- [x] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

vm-image-spec.yaml

@@ -194,7 +194,7 @@ files:
 
       - metric_name: pg_stats_userdb
         type: gauge
-        help: 'Stats for the oldest non-system db'
+        help: 'Stats for several oldest non-system dbs'
         key_labels:
           - datname
         value_label: kind
@@ -205,9 +205,8 @@ files:
           - inserted
           - updated
           - deleted
-        # We export stats for only one non-system database. Without this limit
+        # We export stats for 10 non-system database. Without this limit
         # it is too easy to abuse the system by creating lots of databases.
-        # We can try lifting this limit in the future after we understand the needs better.
         query: |
           select pg_database_size(datname) as db_size, deadlocks,
                  tup_inserted as inserted, tup_updated as updated, tup_deleted as deleted,
@@ -218,7 +217,7 @@ files:
                from pg_database
               where datname <> 'postgres' and not datistemplate
               order by oid
-              limit 1
+              limit 10
            );
 
       - metric_name: max_cluster_size