Skip language tag for external spans

lib/ddtrace/runtime/metrics.rb

@@ -1,3 +1,4 @@
+require 'ddtrace/ext/integration'
 require 'ddtrace/ext/runtime'
 
 require 'ddtrace/metrics'
@@ -25,8 +26,11 @@ def associate_with_span(span)
         # Register service as associated with metrics
         register_service(span.service) unless span.service.nil?
 
-        # Tag span with language and runtime ID for association with metrics
-        span.set_tag(Ext::Runtime::TAG_LANG, Runtime::Identity.lang)
+        # Tag span with language and runtime ID for association with metrics.
+        # We only tag spans that performed internal application work.
+        unless span.get_tag(Datadog::Ext::Integration::TAG_PEER_SERVICE)
+          span.set_tag(Ext::Runtime::TAG_LANG, Runtime::Identity.lang)
+        end
       end
 
       # Associate service with runtime metrics

spec/ddtrace/contrib/rack/configuration_spec.rb

@@ -77,8 +77,7 @@
         expect(queue_span.span_type).to eq('proxy')
         expect(queue_span.service).to eq(Datadog.configuration[:rack][:web_service_name])
         expect(queue_span.start_time.to_i).to eq(queue_time)
-        # Queue span gets tagged for runtime metrics because its a local root span.
-        # TODO: It probably shouldn't get tagged like this in the future; it's not part of the runtime.
+        expect(queue_span.get_tag(Datadog::Ext::Runtime::TAG_LANG)).to be_nil
 
         expect(rack_span.name).to eq('rack.request')
         expect(rack_span.span_type).to eq('web')

spec/ddtrace/runtime/metrics_spec.rb

@@ -24,21 +24,31 @@
 
   describe '#associate_with_span' do
     subject(:associate_with_span) { runtime_metrics.associate_with_span(span) }
-    let(:span) { instance_double(Datadog::Span, service: service) }
+    let(:span) { Datadog::Span.new(nil, 'dummy', service: service) }
     let(:service) { 'parser' }
 
     context 'when enabled' do
       before do
         runtime_metrics.enabled = true
 
-        expect(span).to receive(:set_tag)
-          .with(Datadog::Ext::Runtime::TAG_LANG, Datadog::Runtime::Identity.lang)
-
         associate_with_span
       end
 
-      it 'registers the span\'s service' do
-        expect(runtime_metrics.default_metric_options[:tags]).to include("service:#{service}")
+      context 'with internal span' do
+        it 'registers the span\'s service' do
+          expect(runtime_metrics.default_metric_options[:tags]).to include("service:#{service}")
+          expect(span.get_tag(Datadog::Ext::Runtime::TAG_LANG)).to eq(Datadog::Runtime::Identity.lang)
+        end
+      end
+
+      context 'with external resource span' do
+        let(:span) do
+          super().tap { |s| s.set_tag(Datadog::Ext::Integration::TAG_PEER_SERVICE, 'peer-service-name') }
+        end
+
+        it "doesn't tag as an internal language span" do
+          expect(span.get_tag(Datadog::Ext::Runtime::TAG_LANG)).to be nil
+        end
       end
     end