anchor.diff:  %% isn't a list.<br/>
   %% Raises `{invalid_label_name, Name, Message}' error if `Name' isn't a valid
   %% label name.<br/>
  -%% Raises `{invalid_value_error, Value, MessagE}' error if `duration_unit' is
  -%% unknown or doesn't match metric name.<br/>
   %% @end
   declare(Spec) ->
     prometheus_metric:insert_mf(?TABLE, ?MODULE, Spec).
  
positive.code: 
  %% @private
  collect_mf(Registry, Callback) ->
    [Callback(create_boolean(Name, Help, {Labels, Registry, DU})) ||
      [Name, {Labels, Help}, _, DU, _] <- prometheus_metric:metrics(?TABLE,
                                                                    Registry)],
    ok.
  
  %% @private
  collect_metrics(Name, {Labels, Registry, DU}) ->
    [prometheus_model_helpers:boolean_metric(
       lists:zip(Labels, LabelValues),
       prometheus_time:maybe_convert_to_du(DU, Value)) ||
      [LabelValues, Value] <- ets:match(?TABLE, {{Registry, Name, '$1'}, '$2'})].
  
  %%====================================================================
  
negative.code: %% isn't a list.<br/>
  %% Raises `{invalid_label_name, Name, Message}' error if `Name' isn't a valid
  %% label name.<br/>
  %% Raises `{mf_already_exists, {Registry, Name}, Message}' error if a boolean
  %% with the same `Spec' already exists.
  %% @end
  