Fix Rack integration to tag Content-Type and Content-Length headers (#4359)

The Rack spec says:

> The environment must not contain the keys HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH (use the versions without HTTP_).

See https://github.com/rack/rack/blob/e217a399eb116362710aac7c5b8dc691ea2189b3/SPEC.rdoc?plain=1#L119-L121

Co-authored-by: Marco Costa <[email protected]>

Author: ybiquitous

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

@@ -31,7 +31,17 @@ def key?(header_name)
           end
 
           def self.to_rack_header(name)
-            "HTTP_#{name.to_s.upcase.gsub(/[-\s]/, '_')}"
+            key = name.to_s.upcase.gsub(/[-\s]/, '_')
+            case key
+            when 'CONTENT_TYPE', 'CONTENT_LENGTH'
+              # NOTE: The Rack spec says:
+              # > The environment must not contain the keys HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH
+              # > (use the versions without HTTP_).
+              # See https://github.com/rack/rack/blob/e217a399eb116362710aac7c5b8dc691ea2189b3/SPEC.rdoc?plain=1#L119-L121
+              key
+            else
+              "HTTP_#{key}"
+            end
           end
         end
       end

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

@@ -32,5 +32,24 @@
         expect(collection.get('X-Forwarded-For')).to be_nil
       end
     end
+
+    context 'when Content-Type and Content-Length headers exist in env' do
+      let(:env) do
+        {
+          'CONTENT_TYPE' => 'application/json',
+          'CONTENT_LENGTH' => '120'
+        }
+      end
+
+      it 'returns header value' do
+        expect(collection.get('Content-Type')).to eq('application/json')
+        expect(collection.get('Content-Length')).to eq('120')
+      end
+
+      it 'returns header value regardless of letter casing in the name' do
+        expect(collection.get('content-type')).to eq('application/json')
+        expect(collection.get('content-length')).to eq('120')
+      end
+    end
   end
 end