anchor.diff:  
   -spec summary_metric(Spec) -> prometheus_model:'Metric'() when
       Spec   :: {Labels, Count, Sum} | {Count, Sum},
  -    Labels :: [{_, _}],                         % FIXME: refine
  +    Labels :: [label()],
       Count  :: non_neg_integer(),
       Sum    :: non_neg_integer().
   summary_metric({Labels, Count, Sum}) -> summary_metric(Labels, Count, Sum);
  
positive.code:   histogram_metric([], Buckets, Count, Sum).
  
  -spec histogram_metric(Labels, Buckets, Count, Sum) -> Metric when
      Labels  :: [{_, _}],                        % FIXME: refine
      Buckets :: [{Bound, Count}],
      Bound   :: prometheus_buckets:bucket_bound(),
      Count   :: non_neg_integer(),
  
negative.code: 
  -spec counter_metric(Value) -> prometheus_model:'Metric'() when
      Value  :: {Labels, Val} | {Val} | Val,
      Labels :: [label()],
      Val    :: non_neg_integer().
  counter_metric({Labels, Value}) -> counter_metric(Labels, Value);
  counter_metric({Value})         -> counter_metric([], Value);
  counter_metric(Value)           -> counter_metric([], Value).
  
  -spec counter_metric(Labels, Value) -> prometheus_model:'Metric'() when
      Labels :: [label()],
      Value  :: non_neg_integer().
  counter_metric(Labels, Value) ->
    #'Metric'{label   = label_pairs(Labels),
  