fix grape leading slash resource issue

lib/datadog/tracing/contrib/grape/endpoint.rb

@@ -268,7 +268,7 @@ def endpoint_expand_path(endpoint)
               namespace = endpoint.routes.first && endpoint.routes.first.namespace || ''
 
               parts = (namespace.split('/') + route_path).reject { |p| p.blank? || p.eql?('/') }
-              parts.join('/').prepend('/')
+              parts.join('/').delete_prefix('/').prepend('/')
             end
 
             def service_name

spec/datadog/tracing/contrib/grape/tracer_spec.rb

@@ -94,6 +94,10 @@
             'OK'
           end
         end
+
+        get '/some/nested/path/:uuid' do
+          'nested-path'
+        end
       end
     )
   end
@@ -719,6 +723,19 @@
       end
     end
 
+    describe 'nested path resource' do
+      subject(:response) { get '/some/nested/path/:uuid' }
+
+      before do
+        is_expected.to be_ok
+      end
+
+      it 'sets the resource correctly' do
+        expect(trace.name).to eq('grape.endpoint_run')
+        expect(trace.resource).to eq('TestingAPI GET /some/nested/path/:uuid')
+      end
+    end
+
     context 'when tracing is disabled' do
       subject(:response) { get '/base/success' }