add prometheus variable labels method(LabelVec,WithLabelValues) support

[gersure]

we can use like this:

auto& family = BuildHistogram()
          .Name(name)
          .Help(help)
          .Labels(const_labels)
          .LabelNamesVec(variable_labels)
          .BucketBoundaries({Histogram::BucketBoundaries{1, 2}})
          .Register(registry);
  family.WithLabelValues(variable_values1).Observe(1.0);
  family.WithLabelValues(variable_values2).Observe(2.0);


  auto& family = BuildSummary()
          .Name(name)
          .Help(help)
          .Labels(const_labels)
          .LabelNamesVec(variable_labels)
          .Quantiles(Summary::Quantiles{})
          .Register(registry);
  family.WithLabelValues(variable_values1).Observe(1);
  family.WithLabelValues(variable_values2).Observe(2);