Swap active/inactive in the gatherer

breakdown.go

@@ -259,16 +259,17 @@ func (m *breakdownMetricsMap) record(k breakdownMetricsKey, bt breakdownTiming)
 	return true
 }
 
-func (m *breakdownMetrics) swap() {
+// gather is called by builtinMetricsGatherer to gather breakdown metrics.
+func (m *breakdownMetrics) gather(out *Metrics) {
+	// Hold m.mu only long enough to swap m.active and m.inactive.
+	// This will be blocked up metric updates, but that's OK; only
+	// metrics gathering will be delayed. After swapping we do not
+	// need to hold m.mu, since nothing concurrently accesses
+	// m.inactive while the gatherer is iterating over it.
 	m.mu.Lock()
 	m.active, m.inactive = m.inactive, m.active
 	m.mu.Unlock()
-}
 
-// gather is called by builtinMetricsGatherer to gather breakdown metrics.
-func (m *breakdownMetrics) gather(out *Metrics) {
-	// It is not necessary to hold m.mu, since nothing concurrently
-	// accesses m.inactive while the gatherer is iterating over it.
 	for hash, entries := range m.inactive.m {
 		for _, entry := range entries {
 			if entry.transaction.count > 0 {

tracer.go

@@ -794,7 +794,6 @@ func (t *Tracer) loop() {
 		if gatherMetrics {
 			gatheringMetrics = true
 			metrics.disabled = cfg.disabledMetrics
-			t.breakdownMetrics.swap()
 			t.gatherMetrics(ctx, cfg.metricsGatherers, &metrics, cfg.logger, gatheredMetrics)
 			if cfg.logger != nil {
 				cfg.logger.Debugf("gathering metrics")