Add http.request_id tag to Rack spans

lib/ddtrace/contrib/rack/middlewares.rb

@@ -87,6 +87,7 @@ def set_request_tags!(request_span, env, status, headers, response)
           # we prefer using the `REQUEST_URI` if this is available.
           # NOTE: `REQUEST_URI` is Rails specific and may not apply for other frameworks
           url = env['REQUEST_URI'] || env['PATH_INFO']
+          request_id = get_request_id(headers, env)
 
           request_span.resource ||= resource_name_for(env, status)
           if request_span.get_tag(Datadog::Ext::HTTP::METHOD).nil?
@@ -110,13 +111,24 @@ def set_request_tags!(request_span, env, status, headers, response)
           if request_span.get_tag(Datadog::Ext::HTTP::STATUS_CODE).nil? && status
             request_span.set_tag(Datadog::Ext::HTTP::STATUS_CODE, status)
           end
+          if request_span.get_tag(Datadog::Ext::HTTP::REQUEST_ID).nil? && request_id
+            request_span.set_tag(Datadog::Ext::HTTP::REQUEST_ID, request_id)
+          end
 
           # detect if the status code is a 5xx and flag the request span as an error
           # unless it has been already set by the underlying framework
           if status.to_s.start_with?('5') && request_span.status.zero?
             request_span.status = 1
           end
         end
+
+        # If Rails is present, it will sanitize & use the Request ID header,
+        # or generate a UUID if no request ID header is present, then set that as headers['X-Request-Id'].
+        # Othewise use whatever Rack variables are present (they should all be the same.)
+        def get_request_id(headers, env)
+          headers ||= {}
+          headers['X-Request-Id'] || headers['X-Request-ID'] || env['HTTP_X_REQUEST_ID']
+        end
       end
     end
   end

lib/ddtrace/ext/http.rb

@@ -6,6 +6,7 @@ module HTTP
       URL = 'http.url'.freeze
       BASE_URL = 'http.base_url'.freeze
       METHOD = 'http.method'.freeze
+      REQUEST_ID = 'http.request_id'.freeze
       STATUS_CODE = 'http.status_code'.freeze
       ERROR_RANGE = 500...600
     end

test/contrib/rack/middleware_test.rb

@@ -1,3 +1,4 @@
+require 'securerandom'
 require 'contrib/rack/helpers'
 
 # rubocop:disable Metrics/ClassLength
@@ -283,4 +284,26 @@ def test_request_middleware_custom_service
     assert_equal(0, span.status)
     assert_nil(span.parent)
   end
+
+  def test_request_middleware_request_id
+    request_id = SecureRandom.uuid
+    get '/success/', {}, 'HTTP_X_REQUEST_ID' => request_id
+    assert last_response.ok?
+
+    spans = @tracer.writer.spans
+    assert_equal(1, spans.length)
+
+    span = spans.first
+    assert_equal('rack.request', span.name)
+    assert_equal('http', span.span_type)
+    assert_equal('rack', span.service)
+    assert_equal('GET 200', span.resource)
+    assert_equal('GET', span.get_tag('http.method'))
+    assert_equal('200', span.get_tag('http.status_code'))
+    assert_equal('/success/', span.get_tag('http.url'))
+    assert_equal('http://example.org', span.get_tag('http.base_url'))
+    assert_equal(request_id, span.get_tag('http.request_id'))
+    assert_equal(0, span.status)
+    assert_nil(span.parent)
+  end
 end