Use Sentry.with_child_span in the tracing subscribers

sentry-rails/lib/sentry/rails/tracing/abstract_subscriber.rb

@@ -40,15 +40,12 @@ def subscribe_to_event(event_names)
           def record_on_current_span(duration:, **options)
             return unless options[:start_timestamp]
 
-            scope = Sentry.get_current_scope
-            transaction = scope.get_transaction
-            return unless transaction && transaction.sampled
-
-            span = transaction.start_child(**options)
-            # duration in ActiveSupport is computed in millisecond
-            # so we need to covert it as second before calculating the timestamp
-            span.set_timestamp(span.start_timestamp + duration / 1000)
-            yield(span) if block_given?
+            Sentry.with_child_span(**options) do |child_span|
+              # duration in ActiveSupport is computed in millisecond
+              # so we need to covert it as second before calculating the timestamp
+              child_span.set_timestamp(child_span.start_timestamp + duration / 1000)
+              yield(child_span) if block_given?
+            end
           end
         end
       end