Add Rails cache store instrumentation filter

docs/GettingStarted.md

@@ -552,6 +552,7 @@ cache.read('city')
 | --------------- | - | -------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------- |
 | `enabled` | `DD_TRACE_ACTIVE_SUPPORT_ENABLED` | `Bool` | Whether the integration should create spans. | `true` |
 | `cache_service` | | `String` | Name of application running the `active_support` instrumentation. May be overridden by `global_default_service_name`. [See _Additional Configuration_ for more details](#additional-configuration) | `active_support-cache` |
+| `cache_store` | | `Array` | Specifies which cache stores to instrument. Accepts a list of store names (e.g. `memory_store`, `file_store`, or symbols like `:file_store`). If set, only the listed stores will be traced. By default (`nil`), it traces all stores. | `nil` |
 
 ### AWS
 

lib/datadog/tracing/contrib/active_support/cache/events/cache.rb

@@ -45,9 +45,15 @@ def span_options
                 'cache_write_multi.active_support' => { resource: Ext::RESOURCE_CACHE_MSET, multi_key: true }
               }.freeze
 
-              def trace?(event, _payload)
+              def trace?(event, payload)
                 return false if !Tracing.enabled? || !configuration.enabled
 
+                if (cache_store = configuration[:cache_store])
+                  store = cache_backend(payload[:store])
+
+                  return false unless cache_store.include?(store)
+                end
+
                 # DEV-3.0: Backwards compatibility code for the 2.x gem series.
                 # DEV-3.0: See documentation at {Datadog::Tracing::Contrib::ActiveSupport::Cache::Instrumentation}
                 # DEV-3.0: for the complete information about this backwards compatibility code.

lib/datadog/tracing/contrib/active_support/configuration/settings.rb

@@ -49,6 +49,19 @@ class Settings < Contrib::Configuration::Settings
                 o.default true
               end
             end
+
+            # Specifies which cache stores to trace.
+            # Accepts a list, with the same format as `config.cache_store`
+            # (e.g. `memory_store`, `file_store`, or symbols like `:file_store`).
+            # Defaults to `nil`, which traces all cache stores.
+            # @see https://github.com/rails/rails/blob/b7520a13adda46c0cc5f3fb4a4c1726633af2bba/guides/source/caching_with_rails.md?plain=1#L576-L582
+            option :cache_store do |o|
+              o.type :array, nilable: true
+              o.default nil
+              o.after_set do |stores|
+                stores&.map!(&:to_s) # Convert symbols to strings to match the Rails configuration format
+              end
+            end
           end
         end
       end

spec/datadog/tracing/contrib/rails/cache_spec.rb

@@ -200,6 +200,59 @@
         .to eq('cache')
     end
 
+    context 'when :cache_store config includes the backend' do
+      before do
+        Datadog.configuration.tracing[:active_support][:cache_store] = ['other_store', 'file_store']
+      end
+
+      it 'creates a span' do
+        write
+        expect(span.get_tag('rails.cache.backend')).to eq('file_store')
+      end
+
+      context 'as a symbol' do
+        before do
+          Datadog.configuration.tracing[:active_support][:cache_store] = [:file_store]
+        end
+
+        it 'creates a span' do
+          write
+          expect(span.get_tag('rails.cache.backend')).to eq('file_store')
+        end
+      end
+    end
+
+    context 'when :cache_store config does not include the backend' do
+      before do
+        Datadog.configuration.tracing[:active_support][:cache_store] = ['other_store']
+      end
+
+      it 'creates a span for the non-excluded backend' do
+        expect { write }.to_not(change { fetch_spans.size })
+      end
+
+      context 'with an empty array' do
+        before do
+          Datadog.configuration.tracing[:active_support][:cache_store] = []
+        end
+
+        it 'does not create a span' do
+          expect { write }.to_not(change { fetch_spans.size })
+        end
+      end
+    end
+
+    context 'when :cache_store config is not set' do
+      before do
+        Datadog.configuration.tracing[:active_support].reset!
+      end
+
+      it 'creates a span' do
+        write
+        expect(span.get_tag('rails.cache.backend')).to eq('file_store')
+      end
+    end
+
     context 'with custom cache_service' do
       before do
         Datadog.configuration.tracing[:active_support][:cache_service] = 'service-cache'