Test: wait for metrics

(cherry picked from commit e1c22a0)
(cherry picked from commit c831b4d)

Author: dcorbacho

deps/rabbitmq_prometheus/test/prometheus_rabbitmq_federation_collector_SUITE.erl

@@ -101,8 +101,10 @@ single_link_then_second_added(Config) ->
               timer:sleep(3000),
               [_L1] = rabbit_ct_broker_helpers:rpc(Config, 0,
                                                    rabbit_federation_status, status, []),
-              MFs = get_metrics(Config),
-              [?ONE_RUNNING_METRIC] = MFs,
+              rabbit_ct_helpers:eventually(?_assertEqual([?ONE_RUNNING_METRIC],
+                                                         get_metrics(Config)),
+                                           500,
+                                           5),
               maybe_declare_queue(Config, Ch, q(<<"fed.downstream2">>, [{<<"x-queue-type">>, longstr, <<"classic">>}])),
               %% here we race against queue.declare... most of the times there is going to be
               %% new status=starting metric. In this case we wait a bit more for running=2.