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.

libs/compute_api/src/responses.rs

@@ -1,6 +1,5 @@
 //! Structs representing the JSON formats used in the compute_ctl's HTTP API.
 
-use std::collections::HashSet;
 use std::fmt::Display;
 
 use chrono::{DateTime, Utc};
@@ -163,8 +162,9 @@ pub enum ControlPlaneComputeStatus {
 #[derive(Clone, Debug, Default, Serialize)]
 pub struct InstalledExtension {
     pub extname: String,
-    pub versions: HashSet<String>,
+    pub version: String,
     pub n_databases: u32, // Number of databases using this extension
+    pub owned_by_superuser: String,
 }
 
 #[derive(Clone, Debug, Default, Serialize)]