feat(active-job): Add performance metrics to process span

Record cpu_time, idle_time, gc_time, and allocations as span attributes
on the perform.active_job process span, mirroring the metrics that
ActiveSupport::Notifications::Event captures internally.

Author: bdewater-thatch

instrumentation/active_job/lib/opentelemetry/instrumentation/active_job/handlers/perform.rb

@@ -37,6 +37,37 @@ def start_span(name, _id, payload)
             { span: span, ctx_token: attach_consumer_context(span, parent_context) }
           end
 
+          # Overrides `Default#start` to also snapshot performance metrics at job start
+          #
+          # @param name [String] of the Event
+          # @param id [String] of the event
+          # @param payload [Hash] containing job run information
+          # @return [Hash] the payload passed as a method argument
+          def start(name, id, payload)
+            payload.merge!(__otel: start_span(name, id, payload), __otel_metrics: snapshot_metrics)
+          rescue StandardError => e
+            OpenTelemetry.handle_error(exception: e)
+          end
+
+          # Overrides `Default#finish` to record performance metrics on the span
+          #
+          # @param _name [String] of the Event (unused)
+          # @param _id [String] of the event (unused)
+          # @param payload [Hash] containing job run information
+          def finish(_name, _id, payload)
+            otel = payload.delete(:__otel)
+            metrics_start = payload.delete(:__otel_metrics)
+            span = otel&.fetch(:span)
+            token = otel&.fetch(:ctx_token)
+
+            record_metrics(span, metrics_start) if span && metrics_start
+            on_exception(payload[:error] || payload[:exception_object], span)
+          rescue StandardError => e
+            OpenTelemetry.handle_error(exception: e)
+          ensure
+            finish_span(span, token)
+          end
+
           # This method attaches a span to multiple contexts:
           # 1. Registers the ingress span as the top level ActiveJob span.
           #    This is used later to enrich the ingress span in children, e.g. setting span status to error when a child event like `discard` terminates due to an error
@@ -50,6 +81,56 @@ def attach_consumer_context(span, parent_context)
 
             OpenTelemetry::Context.attach(internal_context)
           end
+
+          private
+
+          def snapshot_metrics
+            {
+              monotonic_time: Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_millisecond),
+              cpu_time: now_cpu,
+              gc_time: now_gc,
+              allocations: GC.stat(:total_allocated_objects)
+            }
+          end
+
+          def record_metrics(span, start)
+            finish = snapshot_metrics
+
+            duration    = finish[:monotonic_time] - start[:monotonic_time]
+            cpu_time    = finish[:cpu_time] - start[:cpu_time]
+            idle_time   = [duration - cpu_time, 0.0].max
+            gc_time     = (finish[:gc_time] - start[:gc_time]) / 1_000_000.0
+            allocations = finish[:allocations] - start[:allocations]
+
+            span.set_attributes(
+              'messaging.active_job.job.cpu_time' => cpu_time,
+              'messaging.active_job.job.idle_time' => idle_time,
+              'messaging.active_job.job.gc_time' => gc_time,
+              'messaging.active_job.job.allocations' => allocations
+            )
+          end
+
+          if GC.respond_to?(:total_time)
+            def now_gc
+              GC.total_time
+            end
+          else
+            def now_gc
+              0
+            end
+          end
+
+          begin
+            Process.clock_gettime(Process::CLOCK_THREAD_CPUTIME_ID, :float_millisecond)
+
+            def now_cpu
+              Process.clock_gettime(Process::CLOCK_THREAD_CPUTIME_ID, :float_millisecond)
+            end
+          rescue StandardError
+            def now_cpu
+              0.0
+            end
+          end
         end
       end
     end

instrumentation/active_job/test/opentelemetry/instrumentation/active_job/handlers/perform_test.rb

@@ -101,6 +101,40 @@
   end
 
   describe 'attributes' do
+    describe 'performance metrics' do
+      it 'records allocations on the process span' do
+        TestJob.perform_now
+
+        _(process_span.attributes['messaging.active_job.job.allocations']).must_be :>, 0
+      end
+
+      it 'records cpu_time on the process span' do
+        TestJob.perform_now
+
+        _(process_span.attributes['messaging.active_job.job.cpu_time']).must_be :>=, 0.0
+      end
+
+      it 'records idle_time on the process span' do
+        TestJob.perform_now
+
+        _(process_span.attributes['messaging.active_job.job.idle_time']).must_be :>=, 0.0
+      end
+
+      it 'records gc_time on the process span' do
+        TestJob.perform_now
+
+        _(process_span.attributes['messaging.active_job.job.gc_time']).must_be :>=, 0.0
+      end
+
+      it 'does not record performance metrics on the publish span' do
+        TestJob.perform_later
+
+        %w[allocations cpu_time idle_time gc_time].each do |metric|
+          _(publish_span.attributes["messaging.active_job.job.#{metric}"]).must_be_nil
+        end
+      end
+    end
+
     describe 'active_job.priority' do
       it 'is unset for unprioritized jobs' do
         TestJob.perform_later