code: func lint(mf dto.MetricFamily) []Problem {
  	fns := []func(mf dto.MetricFamily) []Problem{
  		lintHelp,
  		lintMetricUnits,
  		lintCounter,
  		lintHistogramSummaryReserved,
  	}
  
  	var problems []Problem
  	for _, fn := range fns {
  		problems = append(problems, fn(mf)...)
  	}
  
  	// TODO(mdlayher): lint rules for specific metrics types.
  	return problems
  }
docs: // lint is the entry point for linting a single metric.