message: Fix race on system time in unit test
diff: @@ -65,13 +65,14 @@ some_request_duration_seconds_bucket{x="get",le="+Inf"} 3 42000
  some_request_duration_seconds_sum{x="get"} 14 42000
  some_request_duration_seconds_count{x="get"} 3 42000
  )"_sv);
  - CAF_MESSAGE("multiple runs generate the same output");
  + CAF_MESSAGE("multiple runs with the same timestamp generate the same output");
  + auto ts = make_timestamp();
  std::string res1;
  {
  - auto buf = exporter.collect_from(registry);
  + auto buf = exporter.collect_from(registry, ts);
  res1.assign(buf.begin(), buf.end());
  }
  - CAF_CHECK_EQUAL(res1, exporter.collect_from(registry));
  + CAF_CHECK_EQUAL(res1, exporter.collect_from(registry, ts));
  }
  CAF_TEST_FIXTURE_SCOPE_END()
  