Add alarms prometheus collector.

close #2653

(cherry picked from commit 9fed915)

Conflicts:
	deps/rabbitmq_prometheus/src/rabbit_prometheus_dispatcher.erl
	release-notes/3.9.4.md

deps/rabbitmq_prometheus/src/collectors/prometheus_rabbitmq_alarm_metrics_collector.erl

@@ -0,0 +1,80 @@
+%% This Source Code Form is subject to the terms of the Mozilla Public
+%% License, v. 2.0. If a copy of the MPL was not distributed with this
+%% file, You can obtain one at https://mozilla.org/MPL/2.0/.
+%%
+%% Copyright (c) 2007-2021 VMware, Inc. or its affiliates.  All rights reserved.
+%%
+-module(prometheus_rabbitmq_alarm_metrics_collector).
+
+-export([register/0, deregister_cleanup/1, collect_mf/2]).
+
+-import(prometheus_model_helpers, [create_mf/4, untyped_metric/1]).
+
+-include_lib("prometheus/include/prometheus.hrl").
+
+-behaviour(prometheus_collector).
+
+-define(METRIC_NAME_PREFIX, "rabbitmq_alarms_").
+
+%%====================================================================
+%% Collector API
+%%====================================================================
+
+register() ->
+    ok = prometheus_registry:register_collector(?MODULE).
+
+deregister_cleanup(_) ->
+    ok.
+
+-spec collect_mf(_Registry, Callback) -> ok
+              when _Registry :: prometheus_registry:registry(),
+                   Callback :: prometheus_collector:callback().
+collect_mf(_Registry, Callback) ->
+    try
+        case rabbit_alarm:get_local_alarms(500) %% TODO: figure out timeout
+        of
+            Alarms when is_list(Alarms) ->
+                ActiveAlarms =
+                    lists:foldl(fun ({{resource_limit, disk, _}, _}, Acc) ->
+                                        maps:put(disk_limit, 1, Acc);
+                                    ({{resource_limit, memory, _}, _}, Acc) ->
+                                        maps:put(memory_limit, 1, Acc);
+                                    ({file_descriptor_limit, _}, Acc) ->
+                                        maps:put(file_descriptor_limit, 1, Acc)
+                                end,
+                                #{},
+                                Alarms),
+
+                Callback(create_mf(?METRIC_NAME(<<"file_descriptor_limit">>),
+                                   <<"is 1 if file descriptor limit alarm is in effect">>,
+                                   untyped,
+                                   [untyped_metric(maps:get(file_descriptor_limit,
+                                                            ActiveAlarms,
+                                                            0))])),
+                Callback(create_mf(?METRIC_NAME(<<"disk_limit">>),
+                                   <<"is 1 if disk alarm is in effect">>,
+                                   untyped,
+                                   [untyped_metric(maps:get(disk_limit, ActiveAlarms, 0))])),
+                Callback(create_mf(?METRIC_NAME(<<"memory_limit">>),
+                                   <<"is 1 if memory alarm is in effect">>,
+                                   untyped,
+                                   [untyped_metric(maps:get(memory_limit, ActiveAlarms, 0))])),
+                ok;
+            Error ->
+                rabbit_log:error("alarm_metrics_collector failed to emit metrics: "
+                                 "rabbitm_alarm:get_local_alarms returned ~p",
+                                 [Error]),
+                %% We are not going to render any alarm metrics here.
+                %% Breaks continuity but at least doesn't crash the
+                %% whole scraping endpoint
+                ok
+        end
+    catch
+        exit:{timeout, _} ->
+            rabbit_log:error("alarm_metrics_collector failed to emit metrics: "
+                             "rabbitm_alarm:get_local_alarms timed out"),
+            %% We are not going to render any alarm metrics here.
+            %% Breaks continuity but at least doesn't crash the
+            %% whole scraping endpoint
+            ok
+    end.

deps/rabbitmq_prometheus/src/rabbit_prometheus_dispatcher.erl

@@ -13,7 +13,9 @@
 
 build_dispatcher() ->
     {ok, _} = application:ensure_all_started(prometheus),
-    prometheus_registry:register_collectors([prometheus_rabbitmq_core_metrics_collector]),
+    prometheus_registry:register_collectors([
+        prometheus_rabbitmq_core_metrics_collector,
+        prometheus_rabbitmq_alarm_metrics_collector]),
     prometheus_registry:register_collectors('per-object', [
         prometheus_vm_system_info_collector,
         prometheus_vm_dist_collector,