Add installed_extensions prometheus metric (#9608)

and add /metrics endpoint to compute_ctl to expose such metrics

metric format example for extension pg_rag
with versions 1.2.3 and 1.4.2
installed in 3 and 1 databases respectively:

neon_extensions_installed{extension="pg_rag", version="1.2.3"} = 3
neon_extensions_installed{extension="pg_rag", version="1.4.2"} = 1

------
infra part: https://github.com/neondatabase/flux-fleet/pull/251
---------

Co-authored-by: Tristan Partin <tristan@neon.tech>

test_runner/fixtures/endpoint/http.py

@@ -46,3 +46,8 @@ class EndpointHttpClient(requests.Session):
         )
         res.raise_for_status()
         return res.json()
+
+    def metrics(self) -> str:
+        res = self.get(f"http://localhost:{self.port}/metrics")
+        res.raise_for_status()
+        return res.text