test(gcp_pubsub): fix flaky test

lib-ee/emqx_ee_bridge/test/emqx_ee_bridge_gcp_pubsub_SUITE.erl

@@ -524,6 +524,29 @@ install_telemetry_handler(TestCase) ->
     end),
     Tid.
 
+wait_until_gauge_is(GaugeName, ExpectedValue, Timeout) ->
+    Events = receive_all_events(GaugeName, Timeout),
+    case lists:last(Events) of
+        #{measurements := #{gauge_set := ExpectedValue}} ->
+            ok;
+        #{measurements := #{gauge_set := Value}} ->
+            ct:fail(
+                "gauge ~p didn't reach expected value ~p; last value: ~p",
+                [GaugeName, ExpectedValue, Value]
+            )
+    end.
+
+receive_all_events(EventName, Timeout) ->
+    receive_all_events(EventName, Timeout, []).
+
+receive_all_events(EventName, Timeout, Acc) ->
+    receive
+        {telemetry, #{name := [_, _, EventName]} = Event} ->
+            receive_all_events(EventName, Timeout, [Event | Acc])
+    after Timeout ->
+        lists:reverse(Acc)
+    end.
+
 wait_telemetry_event(TelemetryTable, EventName, ResourceId) ->
     wait_telemetry_event(TelemetryTable, EventName, ResourceId, #{timeout => 5_000, n_events => 1}).
 
@@ -803,6 +826,8 @@ t_publish_success_batch(Config) ->
         ResourceId,
         #{timeout => 15_000, n_events => NumMessages}
     ),
+    wait_until_gauge_is(queuing, 0, _Timeout = 400),
+    wait_until_gauge_is(inflight, 0, _Timeout = 400),
     assert_metrics(
         #{
             batching => 0,