Fix no-op implementation for ContextManager

hearth/lib/hearth/telemetry/no_op.rb

@@ -61,7 +61,9 @@ def record_exception(exception, attributes: nil); end
     class NoOpContextManager < ContextManagerBase
       def current; end
 
-      def current_span; end
+      def current_span
+        NoOpSpan.new
+      end
 
       def attach(context); end
 

hearth/spec/hearth/telemetry/no_op_spec.rb

@@ -67,5 +67,14 @@ module Telemetry
         end
       end
     end
+
+    describe NoOpContextManager do
+      describe '#current_span' do
+        it 'returns an instance of no-op span' do
+          expect(subject.current_span)
+            .to be_an_instance_of(Hearth::Telemetry::NoOpSpan)
+        end
+      end
+    end
   end
 end