Export db size, deadlocks and changed row metrics (#7050)

## Problem

We want to report metrics for the oldest user database.

vm-image-spec.yaml

@@ -142,6 +142,51 @@ files:
         query: |
           select datname, state, count(*) as count from pg_stat_activity where state <> '' group by datname, state;
 
+      - metric_name: pg_stats_userdb
+        type: gauge
+        help: 'Stats for the oldest non-system db'
+        key_labels:
+          - datname
+        value_label: kind
+        values:
+          - db_size
+          - deadlocks
+          # Rows
+          - inserted
+          - updated
+          - deleted
+        # We export stats for only one 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,
+                 datname
+            from pg_stat_database
+           where datname IN (
+             select datname
+               from pg_database
+              where datname <> 'postgres' and not datistemplate
+              order by oid
+              limit 1
+           );
+
+      - metric_name: max_cluster_size
+        type: gauge
+        help: 'neon.max_cluster_size setting'
+        key_labels:
+        values: [max_cluster_size]
+        query: |
+          select setting::int as max_cluster_size from pg_settings where name = 'neon.max_cluster_size';
+
+      - metric_name: db_total_size
+        type: gauge
+        help: 'Size of all databases'
+        key_labels:
+        values: [total]
+        query: |
+          select sum(pg_database_size(datname)) as total from pg_database;
+
 build: |
   # Build cgroup-tools
   #