Add prometheus_array_format: export metrics as list of terms

Introduces a new format module that returns all collected metrics as a
flat list of Erlang 4-tuples [{Name, Type, Labels, Value}] instead of
a binary, enabling in-process consumption without text/protobuf parsing.

Histogram samples are emitted with _bucket/_sum/_count suffixes; summary
samples emit per-quantile entries plus _sum/_count. Labels are returned
as [{BinaryName, BinaryValue}] proplists.

https://claude.ai/code/session_01EG51CCtD9aMBzrNgtFxGCL

Author: claude

src/formats/prometheus_array_format.erl

@@ -0,0 +1,109 @@
+%% @doc Exports all collected metrics as a flat list of Erlang terms.
+%%
+%% Each metric sample is represented as a 4-tuple:
+%% `{MetricName, Type, Labels, Value}'
+%%
+%% Where:
+%% <ul>
+%%   <li>`MetricName' is a binary (histograms/summaries get `_bucket', `_sum',
+%%       `_count' suffixes)</li>
+%%   <li>`Type' is a lowercase atom: `counter', `gauge', `histogram',
+%%       `summary', or `untyped'</li>
+%%   <li>`Labels' is a proplist `[{BinaryName, BinaryValue}]'</li>
+%%   <li>`Value' is an integer or float</li>
+%% </ul>
+%%
+%% Example:
+%% ```
+%% prometheus_array_format:format().
+%% [{<<"http_requests_total">>, counter, [{<<"method">>, <<"GET">>}], 42},
+%%  {<<"latency_bucket">>,      histogram, [{<<"le">>, <<"1.0">>}],   5},
+%%  {<<"latency_sum">>,         histogram, [],                         2.3},
+%%  {<<"latency_count">>,       histogram, [],                         10}]
+%% '''
+-module(prometheus_array_format).
+
+-include("prometheus_model.hrl").
+
+-export([format/0, format/1]).
+
+%% @doc Exports metrics from the default registry.
+-spec format() -> [{binary(), atom(), [{binary(), binary()}], number()}].
+format() ->
+    format(default).
+
+%% @doc Exports metrics from the given registry.
+-spec format(Registry :: atom()) -> [{binary(), atom(), [{binary(), binary()}], number()}].
+format(Registry) ->
+    MFs = collect_mfs(Registry),
+    lists:flatmap(fun mf_to_tuples/1, MFs).
+
+%%====================================================================
+%% Internal functions
+%%====================================================================
+
+collect_mfs(Registry) ->
+    put(?MODULE, []),
+    Callback = fun(Reg, Collector) ->
+        MFCallback = fun(MF) -> put(?MODULE, [MF | get(?MODULE)]) end,
+        prometheus_collector:collect_mf(Reg, Collector, MFCallback)
+    end,
+    prometheus_registry:collect(Registry, Callback),
+    Result = lists:reverse(get(?MODULE)),
+    erase(?MODULE),
+    Result.
+
+mf_to_tuples(#'MetricFamily'{name = Name, type = Type, metric = Metrics}) ->
+    T = atom_type(Type),
+    NameBin = iolist_to_binary(Name),
+    lists:flatmap(fun(M) -> metric_to_tuples(NameBin, T, M) end, Metrics).
+
+atom_type('COUNTER')   -> counter;
+atom_type('GAUGE')     -> gauge;
+atom_type('HISTOGRAM') -> histogram;
+atom_type('SUMMARY')   -> summary;
+atom_type(_)           -> untyped.
+
+metric_to_tuples(Name, T, #'Metric'{label = Labels, counter = #'Counter'{value = V}}) ->
+    [{Name, T, extract_labels(Labels), V}];
+metric_to_tuples(Name, T, #'Metric'{label = Labels, gauge = #'Gauge'{value = V}}) ->
+    [{Name, T, extract_labels(Labels), V}];
+metric_to_tuples(Name, T, #'Metric'{label = Labels, untyped = #'Untyped'{value = V}}) ->
+    [{Name, T, extract_labels(Labels), V}];
+metric_to_tuples(Name, T, #'Metric'{label = Labels,
+        summary = #'Summary'{sample_count = C, sample_sum = S, quantile = Qs}}) ->
+    Base = extract_labels(Labels),
+    QTerms = [{Name, T, Base ++ [{<<"quantile">>, format_float(Q)}], V}
+              || #'Quantile'{quantile = Q, value = V} <- Qs],
+    QTerms ++ [
+        {<<Name/binary, "_sum">>,   T, Base, S},
+        {<<Name/binary, "_count">>, T, Base, C}
+    ];
+metric_to_tuples(Name, T, #'Metric'{label = Labels,
+        histogram = #'Histogram'{sample_count = C, sample_sum = S, bucket = Bs}}) ->
+    Base = extract_labels(Labels),
+    BTerms = [{<<Name/binary, "_bucket">>, T,
+               Base ++ [{<<"le">>, format_bound(UB)}], CC}
+              || #'Bucket'{upper_bound = UB, cumulative_count = CC} <- Bs],
+    BTerms ++ [
+        {<<Name/binary, "_sum">>,   T, Base, S},
+        {<<Name/binary, "_count">>, T, Base, C}
+    ].
+
+extract_labels(Labels) when is_list(Labels) ->
+    [{iolist_to_binary(N), iolist_to_binary(V)}
+     || #'LabelPair'{name = N, value = V} <- Labels];
+extract_labels(_) ->
+    [].
+
+format_bound(infinity) ->
+    <<"+Inf">>;
+format_bound(N) when is_integer(N) ->
+    integer_to_binary(N);
+format_bound(N) ->
+    float_to_binary(N, [{decimals, 10}, compact]).
+
+format_float(N) when is_integer(N) ->
+    integer_to_binary(N);
+format_float(N) ->
+    float_to_binary(N, [{decimals, 10}, compact]).