Fix instrumentation of custom cache stores

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

@@ -101,7 +101,9 @@ def dd_store_name
 
                 # DEV: String#underscore is available through ActiveSupport, and is
                 # DEV: the exact reverse operation to `#camelize`.
-                @store_name = self.class.name.underscore.match(%r{active_support/cache/(.*)})[1]
+                # DEV: String#demodulize is available through ActiveSupport, and is
+                # DEV: used to remove the module ('*::') part of a constant name.
+                @store_name = self.class.name.demodulize.underscore
               end
             end
 

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

@@ -61,6 +61,26 @@
       end
     end
 
+    context 'with a cache not in the ActiveSupport::Cache:: namespace' do
+      let(:cache_class) { stub_const('My::CustomCache', Class.new(ActiveSupport::Cache::MemoryStore)) }
+      let(:cache) { cache_class.new }
+
+      it 'returns the matching backend type' do
+        subject
+        expect(spans[0].get_tag('rails.cache.backend')).to eq('custom_cache')
+      end
+    end
+
+    context 'with an unnamespaced cache class' do
+      let(:cache_class) { stub_const('CustomCache', Class.new(ActiveSupport::Cache::MemoryStore)) }
+      let(:cache) { cache_class.new }
+
+      it 'returns the matching backend type' do
+        subject
+        expect(spans[0].get_tag('rails.cache.backend')).to eq('custom_cache')
+      end
+    end
+
     it_behaves_like 'measured span for integration', false do
       before { subject }
       let(:span) { spans[0] }