Reduce ActionCable broadcast metric cardinality

Previously, the metrics for Action Cable broadcast included the value
for the broadcasting from the Active Support Notification payload.
This value can have very high cardinality. Now, the metrics do not
include the broadcasting value and are much simpler:
"Ruby/ActionCable/broadcast"

Author: kaylareopelle

CHANGELOG.md

@@ -1,5 +1,11 @@
 # New Relic Ruby Agent Release Notes
 
+## dev
+
+- **Bugfix: Reduce cardinality of ActionCable broadcast metrics**
+
+  Previously, the metrics for ActionCable broadcast included the value for the broadcasting. This value can have very high cardinality. Now, the metrics exclude the broadcasting value and are much simpler: `Ruby/ActionCable/broadcast`. (PR#XXXX)[link]
+
 ## v10.2.0
 
 - **Feature: Introduce Hybrid Agent for OpenTelemetry Tracing Support**

lib/new_relic/agent/instrumentation/action_cable_subscriber.rb

@@ -29,11 +29,11 @@ def transaction_name_from_payload(payload)
         end
 
         def metric_name_from_payload(name, payload)
-          "Ruby/ActionCable/#{metric_name(payload)}/#{action_name(name)}"
+          "Ruby/ActionCable/#{metric_name(payload)}#{action_name(name)}"
         end
 
         def metric_name(payload)
-          payload[:broadcasting] || payload[:channel_class]
+          "#{payload[:channel_class]}/" if payload[:channel_class]
         end
 
         DOT_ACTION_CABLE = '.action_cable'.freeze

test/new_relic/agent/instrumentation/rails/action_cable_subscriber.rb

@@ -126,7 +126,7 @@ def test_does_not_record_unscoped_metrics_nor_create_trace_for_broadcast_outside
           sample = last_transaction_trace
 
           assert_nil sample, 'Did not expect a transaction to be created for broadcast'
-          refute_metrics_recorded ['Ruby/ActionCable/TestBroadcasting/broadcast']
+          refute_metrics_recorded ['Ruby/ActionCable/broadcast']
         end
 
         def test_actual_call_to_broadcast_method_records_segment_in_txn
@@ -136,15 +136,15 @@ def test_actual_call_to_broadcast_method_records_segment_in_txn
             @subscriber.finish('broadcast.action_cable', :id, payload_for_broadcast)
           end
 
-          metric_name = 'Ruby/ActionCable/TestBroadcasting/broadcast'
+          metric_name = 'Ruby/ActionCable/broadcast'
 
           assert_metrics_recorded metric_name
           assert find_node_with_name(last_transaction_trace, metric_name),
             'Could not find a node with desired name.'
         end
 
-        def test_metric_name_correctly_names_payload_for_broadcast
-          assert_equal 'TestBroadcasting', @subscriber.send(:metric_name, payload_for_broadcast)
+        def test_metric_name_returns_nil_for_payload_from_broadcast
+          assert_nil @subscriber.send(:metric_name, payload_for_broadcast)
         end
 
         def test_metric_name_correctly_names_payload_for_channel
@@ -200,10 +200,21 @@ def payload_for_transmit(data = {}, via = nil)
         end
 
         def payload_for_broadcast
-          {
-            broadcasting: 'TestBroadcasting',
-            message: {message: 'test_message'},
-            coder: 'idk-we-dont-save-this'
+          {broadcasting: "tasks_964944192",
+            message:
+            {
+              action: "created",
+               task:
+                {"id" => 629957653,
+                 "name" => "Go for a run",
+                 "description" => "Week 9 C25k",
+                 "created_at" =>
+                  "2026-02-18T21:47:55.096Z",
+                 "updated_at" =>
+                  "2026-02-18T21:47:55.096Z",
+                 "status" => "in_progress",
+                 "group_id" => 964944192}
+            }
           }
         end
       end