Add http.useragent tag

While User-Agent is a header and can be included in the
http.request_headers tag, it is not by default, and even if it were it
oculd be removed from the list by the user.

This unconditionally obtains the request header value and stores it in a
dedicated 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 }