Update compute_installed_extensions metric: (#9891)

add owned_by_superuser field to filter out system extensions.

While on it, also correct related code:
- fix the metric setting: use set() instead of inc() in a loop.
inc() is not idempotent and can lead to incorrect results
if the function called multiple times. Currently it is only called at
compute start, but this will change soon.
- fix the return type of the installed_extensions endpoint
to match the metric. Currently it is only used in the test.

compute_tools/src/http/openapi_spec.yaml

@@ -537,12 +537,14 @@ components:
             properties:
               extname:
                 type: string
-              versions:
-                type: array
+              version:
+                type: string
                 items:
                   type: string
               n_databases:
                 type: integer
+              owned_by_superuser:
+                type: integer
 
     SetRoleGrantsRequest:
       type: object