Move partial trace tagging condition to TraceFormatter

TraceOperation passes its tags unconditionally, therefore TraceSegment
now carries trace tags all the way to TraceFormatter, which has the
responsibility to decide whether to apply them or not to one of the
spans.

lib/datadog/tracing/trace_operation.rb

@@ -412,7 +412,7 @@ def build_trace(spans, partial = false)
           name: @name,
           resource: @resource,
           service: @service,
-          tags: !partial ? meta.merge(metrics) : nil,
+          tags: meta.merge(metrics),
           root_span_id: !partial ? @root_span && @root_span.id : nil
         )
       end

lib/ddtrace/transport/trace_formatter.rb

@@ -65,6 +65,11 @@ def set_resource!
       end
 
       def set_trace_tags!
+        # If the root span wasn't specified, don't set this. We don't want to
+        # misset or overwrite the tags of a span that is in the middle of the
+        # trace.
+        return if !@found_root_span
+
         root_span.set_tags(trace.tags)
       end
 
@@ -168,6 +173,8 @@ def find_root_span(trace)
         root_span_id = trace.send(:root_span_id)
         root_span = trace.spans.find { |s| s.id == root_span_id } if root_span_id
         @found_root_span = !root_span.nil?
+
+        # when root span is not found, fall back to last span (partial flush)
         root_span || trace.spans.last
       end
     end

spec/datadog/tracing/trace_operation_spec.rb

@@ -895,7 +895,7 @@
 
         expect(trace.spans).to have(1).items
         expect(trace.spans.map(&:name)).to include('parent')
-        expect(trace.tags).to_not include('foo' => 'bar')
+        expect(trace.tags).to include('foo' => 'bar')
 
         final_flush = trace_op.flush!
         expect(final_flush.spans).to have(1).items

spec/ddtrace/transport/trace_formatter_spec.rb

@@ -165,6 +165,16 @@
         end
       end
 
+      shared_examples 'root span without generic tags' do
+        context 'metrics' do
+          it { expect(root_span.metrics).to_not include({ 'baz' => 42 }) }
+        end
+
+        context 'meta' do
+          it { expect(root_span.meta).to_not include({ 'foo' => 'bar' }) }
+        end
+      end
+
       context 'with no root span' do
         include_context 'no root span'
 
@@ -193,7 +203,7 @@
           it { expect(root_span.resource).to eq('my.job') }
 
           it_behaves_like 'root span with tags'
-          it_behaves_like 'root span with generic tags'
+          it_behaves_like 'root span without generic tags'
         end
       end
 
@@ -225,7 +235,7 @@
           it { expect(root_span.resource).to eq('my.job') }
 
           it_behaves_like 'root span with tags'
-          it_behaves_like 'root span with generic tags'
+          it_behaves_like 'root span without generic tags'
         end
       end
 
@@ -249,6 +259,16 @@
 
           it_behaves_like 'root span with tags'
         end
+
+        context 'when trace has metadata set with generic tags' do
+          include_context 'trace metadata with tags'
+
+          it { is_expected.to be(trace) }
+          it { expect(root_span.resource).to eq(resource) }
+
+          it_behaves_like 'root span with tags'
+          it_behaves_like 'root span with generic tags'
+        end
       end
     end
   end