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

test_runner/regress/test_installed_extensions.py

@@ -1,6 +1,14 @@
-from logging import info
+from __future__ import annotations
 
-from fixtures.neon_fixtures import NeonEnv
+import time
+from logging import info
+from typing import TYPE_CHECKING
+
+from fixtures.log_helper import log
+from fixtures.metrics import parse_metrics
+
+if TYPE_CHECKING:
+    from fixtures.neon_fixtures import NeonEnv
 
 
 def test_installed_extensions(neon_simple_env: NeonEnv):
@@ -85,3 +93,52 @@ def test_installed_extensions(neon_simple_env: NeonEnv):
             assert ext["n_databases"] == 2
             ext["versions"].sort()
             assert ext["versions"] == ["1.2", "1.3"]
+
+    # check that /metrics endpoint is available
+    # ensure that we see the metric before and after restart
+    res = client.metrics()
+    info("Metrics: %s", res)
+    m = parse_metrics(res)
+    neon_m = m.query_all("installed_extensions", {"extension_name": "neon", "version": "1.2"})
+    assert len(neon_m) == 1
+    for sample in neon_m:
+        assert sample.value == 2
+    neon_m = m.query_all("installed_extensions", {"extension_name": "neon", "version": "1.3"})
+    assert len(neon_m) == 1
+    for sample in neon_m:
+        assert sample.value == 1
+
+    endpoint.stop()
+    endpoint.start()
+
+    timeout = 10
+    while timeout > 0:
+        try:
+            res = client.metrics()
+            timeout = -1
+            if len(parse_metrics(res).query_all("installed_extensions")) < 4:
+                # Assume that not all metrics that are collected yet
+                time.sleep(1)
+                timeout -= 1
+                continue
+        except Exception:
+            log.exception("failed to get metrics, assume they are not collected yet")
+            time.sleep(1)
+            timeout -= 1
+            continue
+
+        assert (
+            len(parse_metrics(res).query_all("installed_extensions")) >= 4
+        ), "Not all metrics are collected"
+
+        info("After restart metrics: %s", res)
+        m = parse_metrics(res)
+        neon_m = m.query_all("installed_extensions", {"extension_name": "neon", "version": "1.2"})
+        assert len(neon_m) == 1
+        for sample in neon_m:
+            assert sample.value == 1
+
+        neon_m = m.query_all("installed_extensions", {"extension_name": "neon", "version": "1.3"})
+        assert len(neon_m) == 1
+        for sample in neon_m:
+            assert sample.value == 1