feat(cloudnative-pg): Metrics for extensions updates (#541)

Updating the image can update the extensions available, but doesn't
change the version installed in the database. We want to expose the
info about an available update to the users.
These are the same changes included in
cloudnative-pg/cloudnative-pg#7195

Signed-off-by: Francesco Canovai <[email protected]>

Author: fcanovai

charts/cloudnative-pg/values.yaml

@@ -649,3 +649,35 @@ monitoringQueriesConfigMap:
         - setting:
             usage: "GAUGE"
             description: "Setting value"
+
+    pg_extensions:
+      query: |
+        SELECT
+         current_database() as datname,
+         name as extname,
+         default_version,
+         installed_version,
+         CASE
+           WHEN default_version = installed_version THEN 0
+           ELSE 1
+        END AS update_available
+        FROM pg_catalog.pg_available_extensions
+        WHERE installed_version IS NOT NULL
+      metrics:
+        - datname:
+            usage: "LABEL"
+            description: "Name of the database"
+        - extname:
+            usage: "LABEL"
+            description: "Extension name"
+        - default_version:
+            usage: "LABEL"
+            description: "Default version"
+        - installed_version:
+            usage: "LABEL"
+            description: "Installed version"
+        - update_available:
+            usage: "GAUGE"
+            description: "An update is available"
+      target_databases:
+        - '*'