Immediately scale up autoscaler for cold-start

When the autoscaler gets a stat to scale a service up from 0 we should
act on this immediately in order to decrease cold-start time.

Closes #2961

pkg/autoscaler/multiscaler.go

@@ -76,6 +76,7 @@ type UniScalerFactory func(*Metric, *DynamicConfig) (UniScaler, error)
 type scalerRunner struct {
 	scaler UniScaler
 	stopCh chan struct{}
+	pokeCh chan struct{}
 
 	// mux guards access to metric
 	mux    sync.RWMutex
@@ -229,6 +230,7 @@ func (m *MultiScaler) createScaler(ctx context.Context, metric *Metric) (*scaler
 		scaler: scaler,
 		stopCh: stopCh,
 		metric: *metric,
+		pokeCh: make(chan struct{}),
 	}
 	runner.metric.Status.DesiredScale = -1
 
@@ -247,6 +249,8 @@ func (m *MultiScaler) createScaler(ctx context.Context, metric *Metric) (*scaler
 				return
 			case <-ticker.C:
 				m.tickScaler(ctx, scaler, scaleChan)
+			case <-runner.pokeCh:
+				m.tickScaler(ctx, scaler, scaleChan)
 			}
 		}
 	}()
@@ -300,6 +304,10 @@ func (m *MultiScaler) RecordStat(key string, stat Stat) {
 	if exists {
 		logger := m.logger.With(zap.String(logkey.Key, key))
 		ctx := logging.WithLogger(context.TODO(), logger)
+
 		scaler.scaler.Record(ctx, stat)
+		if scaler.metric.Status.DesiredScale == 0 && stat.PodName == ActivatorPodName {
+			scaler.pokeCh <- struct{}{}
+		}
 	}
 }