Vectors and Family<T>.Add()

[carlosatrios]

@gjasny In a similar vain to https://github.com/jupp0r/prometheus-cpp/issues/380, are you expected to interact with vectors in a similar way? I can't get the output of Add to play nice with any variation of vector.

  using Families = std::variant<std::reference_wrapper<Family<Counter>>, std::reference_wrapper<Family<Gauge>>, std::reference_wrapper<Family<Histogram>>, std::reference_wrapper<Family<Summary>>>;
  std::map<std::string, Families> metricsMap;

auto& histogram = BuildHistogram().Name("some_histogram").Help("Some Histogram").Register(*registry);
  metricsMap.emplace(histogram.GetName(), histogram);

std::vector<Families> example;
example.push_back(histogram.Add({{"example", "example"}});