Hide drop_unroutable metric behind a feature flag

For mixed version cluster compatibility per
suggestion from @dumbbell.

Pair: @dcorbacho.

Part of rabbitmq/rabbitmq-server#1904.

[#165548314]

3 files changed, 48 insertions, 3 deletions

diff --git a/deps/rabbitmq_management_agent/src/rabbit_mgmt_data.erl b/deps/rabbitmq_management_agent/src/rabbit_mgmt_data.erl
index ca7a554d1b..bbeb6c57d6 100644
--- a/deps/rabbitmq_management_agent/src/rabbit_mgmt_data.erl
+++ b/deps/rabbitmq_management_agent/src/rabbit_mgmt_data.erl
@@ -377,6 +377,19 @@ lookup_all(Table, Ids, SecondKey) ->
     end.
 
 maybe_convert_for_compatibility(Table, Slide)
+  when Table =:= channel_stats_fine_stats orelse
+       Table =:= channel_exchange_stats_fine_stats orelse
+       Table =:= vhost_stats_fine_stats ->
+     ConversionNeeded = rabbit_feature_flags:is_disabled(
+                          drop_unroutable_metric),
+     case ConversionNeeded of
+         false ->
+             Slide;
+         true ->
+             %% drop_drop because the metric is named "drop_unroutable"
+             rabbit_mgmt_data_compat:drop_drop_unroutable_metric(Slide)
+     end;
+maybe_convert_for_compatibility(Table, Slide)
   when Table =:= channel_queue_stats_deliver_stats orelse
        Table =:= channel_stats_deliver_stats orelse
        Table =:= queue_stats_deliver_stats orelse
diff --git a/deps/rabbitmq_management_agent/src/rabbit_mgmt_data_compat.erl b/deps/rabbitmq_management_agent/src/rabbit_mgmt_data_compat.erl
index 22ef01fbd9..850a2d1c02 100644
--- a/deps/rabbitmq_management_agent/src/rabbit_mgmt_data_compat.erl
+++ b/deps/rabbitmq_management_agent/src/rabbit_mgmt_data_compat.erl
@@ -26,8 +26,7 @@ fill_get_empty_queue_metric(Slide) ->
           (Value) when is_tuple(Value) andalso size(Value) =:= 8 ->
               Value;
           (Value) when is_tuple(Value) andalso size(Value) =:= 7 ->
-              %% We want to remote the last element, which is
-              %% the count of basic.get on empty queues.
+              %% Inject a 0 for the new metric
               list_to_tuple(
                 tuple_to_list(Value) ++ [0]);
           (Value) ->
@@ -38,7 +37,7 @@ drop_get_empty_queue_metric(Slide) ->
     exometer_slide:map(
       fun
           (Value) when is_tuple(Value) andalso size(Value) =:= 8 ->
-              %% We want to remote the last element, which is
+              %% We want to remove the last element, which is
               %% the count of basic.get on empty queues.
               list_to_tuple(
                 lists:sublist(
@@ -49,6 +48,33 @@ drop_get_empty_queue_metric(Slide) ->
               Value
       end, Slide).
 
+fill_drop_unroutable_metric(Slide) ->
+    exometer_slide:map(
+      fun
+          (Value) when is_tuple(Value) andalso size(Value) =:= 4 ->
+              Value;
+          (Value) when is_tuple(Value) andalso size(Value) =:= 3 ->
+              %% Inject a 0
+              list_to_tuple(
+                tuple_to_list(Value) ++ [0]);
+          (Value) ->
+              Value
+      end, Slide).
+
+drop_drop_unroutable_metric(Slide) ->
+    exometer_slide:map(
+      fun
+          (Value) when is_tuple(Value) andalso size(Value) =:= 4 ->
+              %% Remote the last element.
+              list_to_tuple(
+                lists:sublist(
+                  tuple_to_list(Value), size(Value) - 1));
+          (Value) when is_tuple(Value) andalso size(Value) =:= 3 ->
+              Value;
+          (Value) ->
+              Value
+      end, Slide).
+
 fill_consumer_active_fields(ConsumersStats) ->
     [case proplists:get_value(active, ConsumerStats) of
          undefined ->
diff --git a/deps/rabbitmq_management_agent/src/rabbit_mgmt_ff.erl b/deps/rabbitmq_management_agent/src/rabbit_mgmt_ff.erl
index c1290b6095..46af69077a 100644
--- a/deps/rabbitmq_management_agent/src/rabbit_mgmt_ff.erl
+++ b/deps/rabbitmq_management_agent/src/rabbit_mgmt_ff.erl
@@ -21,3 +21,9 @@
     #{desc          => "Count AMQP `basic.get` on empty queues in stats",
       stability     => stable
      }}).
+
+-rabbit_feature_flag(
+  {drop_unroutable_metric,
+   #{desc          => "Count unroutable publishes to be dropped in stats",
+     stability     => stable
+    }}).