Mark management metrics collection as deprecated

[Why]
Management metrics collection will be removed in RabbitMQ 4.0. The
prometheus plugin provides a better and more scalable alternative.

[How]
The management metrics collection is marked as deprecated in the code
using the Deprecated features subsystem (based on feature flags). See
pull request #7390 for a description of that subsystem.

To test RabbitMQ behavior as if the feature was removed, the following
configuration setting can be used:
deprecated_features.permit.management_metrics_collection = false

Management metrics collection can be turned off anytime, there are no
conditions to do that.

Once management metrics collection is turned off, the management API
will not report any metrics and the UI will show empty graphs.

Note that given the marketing calendar, the deprecated feature will go
directly from "permitted by default" to "removed" in RabbitMQ 4.0. It
won't go through the gradual deprecation process.

Author: dumbbell

deps/rabbitmq_management/src/rabbit_mgmt_app.erl

@@ -27,10 +27,10 @@
                     {mfa,         {rabbit_mgmt_load_definitions, boot, []}}]}).
 
 start(_Type, _StartArgs) ->
-    case application:get_env(rabbitmq_management_agent, disable_metrics_collector, false) of
-        false ->
-            start();
+    case rabbit_mgmt_agent_config:is_metrics_collector_enabled() of
         true ->
+            start();
+        false ->
             rabbit_log:warning("Metrics collection disabled in management agent, "
                                "management only interface started", []),
             start()

deps/rabbitmq_management/src/rabbit_mgmt_features.erl

@@ -7,14 +7,23 @@
 
 -module(rabbit_mgmt_features).
 
--export([is_op_policy_updating_disabled/0]).
+-export([is_op_policy_updating_disabled/0,
+         are_stats_enabled/0]).
 
 is_op_policy_updating_disabled() ->
     case get_restriction([operator_policy_changes, disabled]) of
         true -> true;
         _ -> false
     end.
 
+are_stats_enabled() ->
+    DisabledFromConf = application:get_env(
+      rabbitmq_management, disable_management_stats, false),
+    case DisabledFromConf of
+        true -> false;
+        _    -> rabbit_mgmt_agent_config:is_metrics_collector_permitted()
+    end.
+
 %% Private
 
 get_restriction(Path) ->

deps/rabbitmq_management/src/rabbit_mgmt_util.erl

@@ -135,8 +135,9 @@ disable_stats(ReqData) ->
                    <<"true">> -> true;
                    _ -> false
                end,
-    MgmtOnly orelse get_bool_env(rabbitmq_management, disable_management_stats, false)
-        orelse get_bool_env(rabbitmq_management_agent, disable_metrics_collector, false).
+    MgmtOnly orelse
+    not rabbit_mgmt_agent_config:is_metrics_collector_enabled() orelse
+    not rabbit_mgmt_features:are_stats_enabled().
 
 enable_queue_totals(ReqData) ->
     EnableTotals = case qs_val(<<"enable_queue_totals">>, ReqData) of

deps/rabbitmq_management_agent/src/rabbit_mgmt_agent_config.erl

@@ -7,6 +7,14 @@
 -module(rabbit_mgmt_agent_config).
 
 -export([get_env/1, get_env/2]).
+-export([is_metrics_collector_enabled/0,
+         is_metrics_collector_permitted/0]).
+
+-rabbit_deprecated_feature(
+   {management_metrics_collection,
+    #{deprecation_phase => permitted_by_default,
+      doc_url => "https://blog.rabbitmq.com/posts/2021/08/4.0-deprecation-announcements/#disable-metrics-delivery-via-the-management-api--ui"
+     }}).
 
 %% some people have reasons to only run with the agent enabled:
 %% make it possible for them to configure key management app
@@ -20,3 +28,15 @@ get_env(Key, Default) ->
     rabbit_misc:get_env(rabbitmq_management, Key,
                         rabbit_misc:get_env(rabbitmq_management_agent, Key,
                                             Default)).
+
+is_metrics_collector_enabled() ->
+    DisabledFromConf = application:get_env(
+      rabbitmq_management_agent, disable_metrics_collector, false),
+    case DisabledFromConf of
+        true -> false;
+        _    -> is_metrics_collector_permitted()
+    end.
+
+is_metrics_collector_permitted() ->
+    FeatureName = management_metrics_collection,
+    rabbit_deprecated_features:is_permitted(FeatureName).

deps/rabbitmq_management_agent/src/rabbit_mgmt_agent_sup.erl

@@ -34,8 +34,8 @@ start_link() ->
 
 
 maybe_enable_metrics_collector() ->
-    case application:get_env(rabbitmq_management_agent, disable_metrics_collector, false) of
-        false ->
+    case rabbit_mgmt_agent_config:is_metrics_collector_enabled() of
+        true ->
             ok = pg:join(?MANAGEMENT_PG_SCOPE, ?MANAGEMENT_PG_GROUP, self()),
             ST = {rabbit_mgmt_storage, {rabbit_mgmt_storage, start_link, []},
                   permanent, ?WORKER_WAIT, worker, [rabbit_mgmt_storage]},
@@ -52,6 +52,6 @@ maybe_enable_metrics_collector() ->
             GC = {rabbit_mgmt_gc, {rabbit_mgmt_gc, start_link, []},
           permanent, ?WORKER_WAIT, worker, [rabbit_mgmt_gc]},
             [ST, MD, GC | MC ++ MGC];
-        true ->
+        false ->
             []
     end.