Add http.useragent tag

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

@@ -139,6 +139,10 @@ def set_request_tags!(trace, request_span, env, status, headers, response, origi
             request_headers_tags = parse_request_headers(request_header_collection)
             response_headers_tags = parse_response_headers(headers || {})
 
+            # request_headers is subject to filtering and configuration so we
+            # get the user agent separately
+            user_agent = parse_user_agent_header(request_header_collection)
+
             # The priority
             # 1. User overrides span.resource
             # 2. Configuration
@@ -205,6 +209,10 @@ def set_request_tags!(trace, request_span, env, status, headers, response, origi
               request_span.set_tag(Tracing::Metadata::Ext::HTTP::TAG_STATUS_CODE, status)
             end
 
+            if request_span.get_tag(Tracing::Metadata::Ext::HTTP::TAG_USER_AGENT).nil? && user_agent
+              request_span.set_tag(Tracing::Metadata::Ext::HTTP::TAG_USER_AGENT, user_agent)
+            end
+
             # Request headers
             request_headers_tags.each do |name, value|
               request_span.set_tag(name, value) if request_span.get_tag(name).nil?
@@ -230,6 +238,10 @@ def configuration
             Datadog.configuration.tracing[:rack]
           end
 
+          def parse_user_agent_header(headers)
+            headers.get(Tracing::Metadata::Ext::HTTP::HEADER_USER_AGENT)
+          end
+
           def parse_request_headers(headers)
             whitelist = configuration[:headers][:request] || []
             whitelist.each_with_object({}) do |header, result|

lib/datadog/tracing/metadata/ext.rb

@@ -63,12 +63,14 @@ module HTTP
           TAG_BASE_URL = 'http.base_url'
           TAG_METHOD = 'http.method'
           TAG_STATUS_CODE = 'http.status_code'
+          TAG_USER_AGENT = 'http.useragent'
           TAG_URL = 'http.url'
           TYPE_INBOUND = AppTypes::TYPE_WEB.freeze
           TYPE_OUTBOUND = 'http'
           TYPE_PROXY = 'proxy'
           TYPE_TEMPLATE = 'template'
           TAG_CLIENT_IP = 'http.client_ip'
+          HEADER_USER_AGENT = 'User-Agent'
 
           # General header functionality
           module Headers

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

@@ -685,6 +685,48 @@
         end
 
         describe 'GET request' do
+          context 'that does not sent user agent' do
+            subject(:response) { get '/headers/', {}, headers }
+
+            let(:headers) do
+              {}
+            end
+
+            before do
+              is_expected.to be_ok
+              expect(spans).to have(1).items
+            end
+
+            it_behaves_like 'a rack GET 200 span'
+
+            it do
+              expect(span.get_tag('http.useragent')).to be nil
+              expect(span.get_tag('http.request.headers.user-agent')).to be nil
+            end
+          end
+
+          context 'that sends user agent' do
+            subject(:response) { get '/headers/', {}, headers }
+
+            let(:headers) do
+              {
+                'HTTP_USER_AGENT' => 'SuperUserAgent',
+              }
+            end
+
+            before do
+              is_expected.to be_ok
+              expect(spans).to have(1).items
+            end
+
+            it_behaves_like 'a rack GET 200 span'
+
+            it do
+              expect(span.get_tag('http.useragent')).to eq('SuperUserAgent')
+              expect(span.get_tag('http.request.headers.user-agent')).to be nil
+            end
+          end
+
           context 'that sends headers' do
             subject(:response) { get '/headers/', {}, headers }