fix(pkg/aggregator): gate hookBatch append on HasSubscribers()

The unconditional append inside sample() was copying a 72-byte
MetricSampleSnapshot struct on every DogStatsD metric even when no
observer was listening. SMP profiler comparison (main vs branch with
0 subscribers) showed +12ms in (*TimeSampler).sample() from this copy
alone.

Gating on HasSubscribers() makes the idle path (0 subscribers) match
main exactly: one atomic read, then nothing. The append only runs when
someone is actually subscribed.

Before: noop/0sub/1sub/5sub all ~82 ns (append unconditional)
After:  noop/0sub ~77 ns; 1sub ~82 ns; 5sub ~86 ns

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: misteriaud

pkg/aggregator/time_sampler.go

@@ -97,14 +97,16 @@ func (s *TimeSampler) sample(metricSample *metrics.MetricSample, timestamp float
 	contextKey := s.contextResolver.trackContext(metricSample, int64(timestamp), filterList)
 	bucketStart := s.calculateBucketStart(timestamp)
 
-	s.hookBatch = append(s.hookBatch, hook.MetricSampleSnapshot{
-		Name:       metricSample.Name,
-		Value:      metricSample.Value,
-		RawTags:    metricSample.Tags,
-		Timestamp:  timestamp,
-		SampleRate: metricSample.SampleRate,
-		ContextKey: uint64(contextKey),
-	})
+	if s.metricHook.HasSubscribers() {
+		s.hookBatch = append(s.hookBatch, hook.MetricSampleSnapshot{
+			Name:       metricSample.Name,
+			Value:      metricSample.Value,
+			RawTags:    metricSample.Tags,
+			Timestamp:  timestamp,
+			SampleRate: metricSample.SampleRate,
+			ContextKey: uint64(contextKey),
+		})
+	}
 
 	switch metricSample.Mtype {
 	case metrics.DistributionType: