add: Log actual host and port of listener connection

Diagnosing problems with listener channel notifications not being handled properly by PostgREST connected to read replicas is difficult. Issues might be related to lost connections and listener not being connected to the right host after failover or database server restarts.
This patch adds logging of actual host:port used by libpq connection opened by the listener. It should make it easier to find out if PostgREST is connected to the right host.

test/io/test_io.py

@@ -1658,6 +1658,23 @@ def test_log_listener_connection_errors(defaultenv):
         )
 
 
+def test_log_listener_connection_start(defaultenv):
+    "The logs should show the listener connection start message in a single line"
+
+    env = {
+        **defaultenv,
+        "PGRST_DB_CHANNEL_ENABLED": "true",
+    }
+
+    with run(env=env, no_startup_stdout=False, wait_for_readiness=True) as postgrest:
+        output = postgrest.read_stdout(nlines=5)
+        assert any(
+            f'Listener connected to "{defaultenv["PGHOST"]}:5432" and listening for database notifications on the "pgrst" channel'
+            in line
+            for line in output
+        )
+
+
 def test_db_pre_config_with_pg_reserved_words(defaultenv):
     "The db-pre-config should not fail unexpectedly when function name is a postgres reserved word"
 

test/observability/ObsHelper.hs

@@ -206,5 +206,5 @@ waitForObs (ObsChan orig copy) t msg f =
 obsDiagMessage :: Observation -> Text
 obsDiagMessage = \case
   (HasqlPoolObs o) -> show o
-  o@(DBListenStart channel) -> constrName o <> show channel
+  o@(DBListenStart host port channel) -> constrName o <> show (host, port, channel)
   o -> constrName o