Add http.route tag setting to rack middleware as fallback

lib/datadog/tracing/contrib/rack/middlewares.rb

@@ -195,6 +195,10 @@ def set_request_tags!(trace, request_span, env, status, headers, response, origi
               request_span.set_tag(Tracing::Metadata::Ext::HTTP::TAG_USER_AGENT, user_agent)
             end
 
+            if request_span.get_tag(Tracing::Metadata::Ext::HTTP::TAG_ROUTE).nil?
+              request_span.set_tag(Tracing::Metadata::Ext::HTTP::TAG_ROUTE, env['PATH_INFO'])
+            end
+
             HeaderTagging.tag_request_headers(request_span, request_header_collection, configuration)
             HeaderTagging.tag_response_headers(request_span, headers, configuration) if headers
 

spec/datadog/tracing/contrib/rack/http_route_spec.rb

@@ -0,0 +1,66 @@
+require 'datadog/tracing/contrib/support/spec_helper'
+
+require 'rack/test'
+
+require 'datadog'
+require 'datadog/tracing/contrib/rack/middlewares'
+
+RSpec.describe 'Rack testing for http.route' do
+  include Rack::Test::Methods
+
+  before do
+    Datadog.configure do |c|
+      c.tracing.instrument :rack
+    end
+  end
+
+  after do
+    Datadog.configuration.tracing[:rack].reset!
+  end
+
+  let(:app) do
+    app = rack_app
+
+    Rack::Builder.new do
+      use Datadog::Tracing::Contrib::Rack::TraceMiddleware
+
+      %w[/ /rack].each do |route|
+        map route do
+          run app
+        end
+      end
+    end.to_app
+  end
+
+  let(:rack_app) do
+    Rack::Builder.new do
+      map '/hello/world' do
+        run ->(_env) { [200, { 'content-type' => 'text/plain' }, 'hello world'] }
+      end
+
+      map '/hello/:id' do
+        run ->(_env) { [200, { 'content-type' => 'text/plain' }, "hello #{params[:id]}"] }
+      end
+    end
+  end
+
+  it 'sets http.route tag on request to base route' do
+    response = get('/hello/world')
+
+    expect(response).to be_ok
+    expect(request_span.get_tag('http.route')).to eq('/hello/world')
+  end
+
+  it 'sets http.route tag on requst to nested app route' do
+    response = get('/rack/hello/world')
+
+    expect(response).to be_ok
+    expect(request_span.get_tag('http.route')).to eq('/rack/hello/world')
+  end
+
+  def request_span
+    spans.detect do |span|
+      span.name == Datadog::Tracing::Contrib::Rack::Ext::SPAN_REQUEST
+    end
+  end
+end