Fix checkpoint metric (#7701)

Split checkpoint_stats into two separate metrics: checkpoints_req and
checkpoints_timed

Fixes commit
https://github.com/neondatabase/neon/commit/21e1a496a3f706097578de396a9107813c541001

---------

Co-authored-by: Peter Bendel <peterbendel@neon.tech>

vm-image-spec.yaml

@@ -278,15 +278,21 @@ files:
               ELSE GREATEST (0, EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()))
             END AS replication_delay_seconds;
 
-      - metric_name: checkpoint_stats
+      - metric_name: checkpoints_req
         type: gauge
-        help: 'Number of requested and scheduled checkpoints'
+        help: 'Number of requested checkpoints'
         key_labels:
-        values:
-          - checkpoints_req
-          - checkpoints_timed
+        values: [checkpoints_req]
         query: |
-          SELECT checkpoints_req, checkpoints_timed FROM pg_stat_bgwriter;
+          SELECT checkpoints_req FROM pg_stat_bgwriter;
+
+      - metric_name: checkpoints_timed
+        type: gauge
+        help: 'Number of scheduled checkpoints'
+        key_labels:
+        values: [checkpoints_timed]
+        query: |
+          SELECT checkpoints_timed FROM pg_stat_bgwriter;
   - filename: neon_collector_autoscaling.yml
     content: |
       collector_name: neon_collector_autoscaling