feat: native metrics with Prometheus support (#772, #560)

Replace external metrics library with hackney-native metrics abstraction:

- Add hackney_metrics_backend behaviour for pluggable backends
- Add hackney_metrics_dummy backend (no-op, default)
- Add hackney_metrics_prometheus backend (opt-in)
- Rewrite hackney_metrics as API facade routing to backends
- Update hackney_manager to use new metrics API (request metrics)
- Update hackney_pool to use new metrics API (pool metrics)
- Remove external metrics library dependency

Metric types fixed (issue #560):
- Pool free_count/in_use_count now use gauges (not histograms)
- Request active count now uses gauge (not counter)

New metric names (Prometheus-compatible):
- hackney_requests_total (counter, labels: host)
- hackney_requests_active (gauge, labels: host)
- hackney_requests_finished_total (counter, labels: host)
- hackney_request_duration_seconds (histogram, labels: host)
- hackney_pool_free_count (gauge, labels: pool)
- hackney_pool_in_use_count (gauge, labels: pool)
- hackney_pool_checkouts_total (counter, labels: pool)

Configuration:
- Default: hackney_metrics_dummy (no-op, zero overhead)
- To enable Prometheus: {hackney, [{metrics_backend, prometheus}]}

Closes #772, closes #560

Author: benoitc

.gitignore

@@ -15,3 +15,4 @@ rebar3.crashdump
 doc/
 priv/*.so
 priv/*.dll
+rebar.lock

rebar.config

@@ -14,6 +14,17 @@
  ]}.
 
 {xref_checks, [undefined_function_calls]}.
+%% Ignore xref warnings for optional prometheus dependency
+{xref_ignores, [
+  {prometheus_counter, inc, 3},
+  {prometheus_counter, declare, 1},
+  {prometheus_gauge, set, 3},
+  {prometheus_gauge, inc, 2},
+  {prometheus_gauge, dec, 2},
+  {prometheus_gauge, declare, 1},
+  {prometheus_histogram, observe, 3},
+  {prometheus_histogram, declare, 1}
+]}.
 
 {cover_enabled, true}.
 {eunit_opts, [verbose]}.
@@ -37,7 +48,6 @@
         {idna, "~>6.1.0"},
         {mimerl, "~>1.4"},
         {certifi, "~>2.15.0"},
-        {metrics, "~>1.0.0"},
         {parse_trans, "3.4.1"},
         {ssl_verify_fun, "~>1.1.0"},
         {unicode_util_compat, "~>0.7.1"}
@@ -85,6 +95,8 @@
     error_handling%,
     %unknown
   ]},
+  %% Exclude prometheus backend - prometheus is an optional dependency
+  {exclude_mods, [hackney_metrics_prometheus]},
   {base_plt_apps, [erts, stdlib, kernel, crypto, runtime_tools]},
   {plt_apps, top_level_deps},
   {plt_extra_apps, []},

rebar.lock

@@ -16,7 +16,6 @@
       mimerl,
       certifi,
       ssl_verify_fun,
-      metrics,
       unicode_util_compat]},
     {included_applications, []},
     {mod, { hackney_app, []}},

src/hackney.app.src

@@ -12,8 +12,7 @@
 
 %% Metrics API
 -export([start_request/1,
-         finish_request/2,
-         get_metrics_engine/0]).
+         finish_request/2]).
 
 %% Backward compatibility API
 -export([get_state/1, async_response_pid/1]).
@@ -23,9 +22,7 @@
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
          terminate/2, code_change/3]).
 
--record(state, {
-    metrics_engine
-}).
+-record(state, {}).
 
 %%====================================================================
 %% API
@@ -41,11 +38,6 @@ start_request(Host) ->
 finish_request(Host, StartTime) ->
     gen_server:cast(?MODULE, {finish_request, Host, StartTime}).
 
-%% @doc Get the current metrics engine.
--spec get_metrics_engine() -> module().
-get_metrics_engine() ->
-    hackney_metrics:get_engine().
-
 %% @doc Check the state of a connection (backward compatibility).
 %% In the old architecture, this tracked request state.
 %% In the new architecture, we simply check if the connection process is alive.
@@ -63,20 +55,15 @@ get_state(ConnPid) when is_pid(ConnPid) ->
 get_state(_) ->
     req_not_found.
 
-%% @doc Check if a connection is in async mode (backward compatibility).
-%% In the old architecture, this returned the async response process PID.
-%% In the new architecture, we check if the connection process is in async mode.
-%% Returns `{error, req_not_async}' if not in async mode.
--spec async_response_pid(pid() | term()) -> {ok, pid()} | {error, req_not_async}.
-async_response_pid(ConnPid) when is_pid(ConnPid) ->
-    case is_process_alive(ConnPid) of
-        false -> {error, req_not_async};
-        true ->
-            case hackney_conn:get_state(ConnPid) of
-                {ok, State} when State =:= receiving; State =:= streaming ->
-                    {ok, ConnPid};
-                _ ->
-                    {error, req_not_async}
+%% @doc Get the async response pid (backward compatibility).
+-spec async_response_pid(pid()) -> {ok, pid()} | {error, req_not_found | req_not_async}.
+async_response_pid(Ref) when is_pid(Ref) ->
+    case get_state(Ref) of
+        req_not_found -> {error, req_not_found};
+        State ->
+            case proplists:get_value(async, State, false) of
+                false -> {error, req_not_async};
+                true -> {ok, Ref}
             end
     end;
 async_response_pid(_) ->
@@ -90,28 +77,27 @@ start_link() ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
 
 init([]) ->
-    %% Initialize metrics
-    Engine = hackney_metrics:get_engine(),
-    _ = metrics:new(Engine, counter, [hackney, nb_requests]),
-    _ = metrics:new(Engine, counter, [hackney, total_requests]),
-    _ = metrics:new(Engine, counter, [hackney, finished_requests]),
-    {ok, #state{metrics_engine = Engine}}.
+    {ok, #state{}}.
 
 handle_call(_Request, _From, State) ->
     {reply, ok, State}.
 
-handle_cast({start_request, Host}, #state{metrics_engine = Engine} = State) ->
-    _ = metrics:increment_counter(Engine, [hackney, Host, nb_requests]),
-    _ = metrics:increment_counter(Engine, [hackney, nb_requests]),
-    _ = metrics:increment_counter(Engine, [hackney, total_requests]),
+handle_cast({start_request, Host}, State) ->
+    HostBin = to_binary(Host),
+    Labels = #{host => HostBin},
+    _ = hackney_metrics:counter_inc(hackney_requests_total, Labels),
+    _ = hackney_metrics:gauge_inc(hackney_requests_active, Labels),
     {noreply, State};
 
-handle_cast({finish_request, Host, StartTime}, #state{metrics_engine = Engine} = State) ->
-    RequestTime = timer:now_diff(os:timestamp(), StartTime) / 1000,
-    _ = metrics:update_histogram(Engine, [hackney, Host, request_time], RequestTime),
-    _ = metrics:decrement_counter(Engine, [hackney, Host, nb_requests]),
-    _ = metrics:decrement_counter(Engine, [hackney, nb_requests]),
-    _ = metrics:increment_counter(Engine, [hackney, finished_requests]),
+handle_cast({finish_request, Host, StartTime}, State) ->
+    HostBin = to_binary(Host),
+    Labels = #{host => HostBin},
+    %% Calculate duration in seconds (Prometheus convention)
+    DurationMicros = timer:now_diff(os:timestamp(), StartTime),
+    DurationSeconds = DurationMicros / 1000000,
+    _ = hackney_metrics:histogram_observe(hackney_request_duration_seconds, Labels, DurationSeconds),
+    _ = hackney_metrics:gauge_dec(hackney_requests_active, Labels),
+    _ = hackney_metrics:counter_inc(hackney_requests_finished_total, Labels),
     {noreply, State};
 
 handle_cast(_Msg, State) ->
@@ -125,3 +111,11 @@ terminate(_Reason, _State) ->
 
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
+
+%%====================================================================
+%% Internal functions
+%%====================================================================
+
+to_binary(Host) when is_binary(Host) -> Host;
+to_binary(Host) when is_list(Host) -> list_to_binary(Host);
+to_binary(Host) when is_atom(Host) -> atom_to_binary(Host, utf8).

src/hackney_manager.erl

@@ -3,7 +3,7 @@
 %%% This file is part of hackney released under the Apache 2 license.
 %%% See the NOTICE for more information.
 %%%
-%%% Copyright (c) 2012-2018 Benoît Chesneau <benoitc@e-engura.org>
+%%% Copyright (c) 2012-2026 Benoît Chesneau <benoitc@e-engura.org>
 %%%
 
 -module(hackney_metrics).
@@ -12,16 +12,122 @@
 %% API
 -export([
   init/0,
-  get_engine/0
+  get_backend/0
 ]).
 
+%% Counter operations
+-export([
+  counter_inc/2,
+  counter_inc/3
+]).
+
+%% Gauge operations
+-export([
+  gauge_set/3,
+  gauge_inc/2,
+  gauge_dec/2
+]).
+
+%% Histogram operations
+-export([
+  histogram_observe/3
+]).
+
+%% Metric declarations
+-export([
+  declare_counter/3,
+  declare_gauge/3,
+  declare_histogram/3,
+  declare_histogram/4,
+  declare_pool_metrics/1
+]).
 
 -include("hackney.hrl").
 
+%% Default duration histogram buckets (in seconds)
+-define(DEFAULT_DURATION_BUCKETS, [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10.0]).
 
+%% @doc Initialize the metrics system.
+%% Determines the backend to use and declares all hackney metrics.
 init() ->
-  Metrics = metrics:init(hackney_util:mod_metrics()),
-  ets:insert(?CONFIG, {mod_metrics, Metrics}).
+  Backend = hackney_util:mod_metrics(),
+  ets:insert(?CONFIG, {metrics_backend, Backend}),
+  declare_metrics(Backend).
+
+%% @doc Get the current metrics backend module.
+get_backend() ->
+  try
+    ets:lookup_element(?CONFIG, metrics_backend, 2)
+  catch
+    error:badarg ->
+      %% ETS table not ready yet, return dummy backend
+      hackney_metrics_dummy
+  end.
+
+%% @doc Increment a counter by 1.
+counter_inc(Name, Labels) ->
+  (get_backend()):counter_inc(Name, Labels).
+
+%% @doc Increment a counter by a value.
+counter_inc(Name, Labels, Value) ->
+  (get_backend()):counter_inc(Name, Labels, Value).
+
+%% @doc Set a gauge to a value.
+gauge_set(Name, Labels, Value) ->
+  (get_backend()):gauge_set(Name, Labels, Value).
+
+%% @doc Increment a gauge by 1.
+gauge_inc(Name, Labels) ->
+  (get_backend()):gauge_inc(Name, Labels).
+
+%% @doc Decrement a gauge by 1.
+gauge_dec(Name, Labels) ->
+  (get_backend()):gauge_dec(Name, Labels).
+
+%% @doc Observe a value for a histogram.
+histogram_observe(Name, Labels, Value) ->
+  (get_backend()):histogram_observe(Name, Labels, Value).
+
+%% @doc Declare a counter metric.
+declare_counter(Name, Help, LabelKeys) ->
+  (get_backend()):declare_counter(Name, Help, LabelKeys).
+
+%% @doc Declare a gauge metric.
+declare_gauge(Name, Help, LabelKeys) ->
+  (get_backend()):declare_gauge(Name, Help, LabelKeys).
+
+%% @doc Declare a histogram metric with default buckets.
+declare_histogram(Name, Help, LabelKeys) ->
+  (get_backend()):declare_histogram(Name, Help, LabelKeys).
+
+%% @doc Declare a histogram metric with custom buckets.
+declare_histogram(Name, Help, LabelKeys, Buckets) ->
+  (get_backend()):declare_histogram(Name, Help, LabelKeys, Buckets).
+
+%% @doc Declare pool-specific metrics.
+%% Called when a new pool is created.
+declare_pool_metrics(_PoolName) ->
+  Backend = get_backend(),
+  %% Only declare once (idempotent for prometheus)
+  Backend:declare_gauge(hackney_pool_free_count,
+    <<"Number of free/available connections in the pool">>, [pool]),
+  Backend:declare_gauge(hackney_pool_in_use_count,
+    <<"Number of connections currently in use">>, [pool]),
+  Backend:declare_counter(hackney_pool_checkouts_total,
+    <<"Total number of connection checkouts">>, [pool]),
+  ok.
 
-get_engine() ->
-  ets:lookup_element(?CONFIG, mod_metrics, 2).
+%% @private
+%% Declare all hackney metrics at startup.
+declare_metrics(Backend) ->
+  %% Request metrics
+  Backend:declare_counter(hackney_requests_total,
+    <<"Total number of HTTP requests started">>, [host]),
+  Backend:declare_gauge(hackney_requests_active,
+    <<"Number of currently active HTTP requests">>, [host]),
+  Backend:declare_counter(hackney_requests_finished_total,
+    <<"Total number of HTTP requests finished">>, [host]),
+  Backend:declare_histogram(hackney_request_duration_seconds,
+    <<"HTTP request duration in seconds">>, [host], ?DEFAULT_DURATION_BUCKETS),
+  %% Pool metrics are declared when pools are created
+  ok.

src/hackney_metrics.erl

@@ -0,0 +1,33 @@
+%%% -*- erlang -*-
+%%%
+%%% This file is part of hackney released under the Apache 2 license.
+%%% See the NOTICE for more information.
+%%%
+%%% Copyright (c) 2012-2026 Benoît Chesneau <benoitc@e-engura.org>
+%%%
+
+-module(hackney_metrics_backend).
+-author("benoitc").
+
+%% Behaviour callbacks for hackney metrics backends
+%%
+%% Implementations must export all callback functions.
+%% See hackney_metrics_dummy for a reference implementation.
+
+%% Counter operations (monotonically increasing)
+-callback counter_inc(Name :: atom(), Labels :: map()) -> ok.
+-callback counter_inc(Name :: atom(), Labels :: map(), Value :: number()) -> ok.
+
+%% Gauge operations (can go up or down)
+-callback gauge_set(Name :: atom(), Labels :: map(), Value :: number()) -> ok.
+-callback gauge_inc(Name :: atom(), Labels :: map()) -> ok.
+-callback gauge_dec(Name :: atom(), Labels :: map()) -> ok.
+
+%% Histogram operations (for timing/distribution measurements)
+-callback histogram_observe(Name :: atom(), Labels :: map(), Value :: number()) -> ok.
+
+%% Metric lifecycle
+-callback declare_counter(Name :: atom(), Help :: binary(), LabelKeys :: [atom()]) -> ok.
+-callback declare_gauge(Name :: atom(), Help :: binary(), LabelKeys :: [atom()]) -> ok.
+-callback declare_histogram(Name :: atom(), Help :: binary(), LabelKeys :: [atom()]) -> ok.
+-callback declare_histogram(Name :: atom(), Help :: binary(), LabelKeys :: [atom()], Buckets :: [number()]) -> ok.