cmd/trace-agent: reduce histogram calls in concentrator

Reduce UDP hits and flooding by calling Histogram only once per flush.

Author: gbbr

cmd/trace-agent/concentrator.go

@@ -142,6 +142,10 @@ func (c *Concentrator) Flush() []model.StatsBucket {
 
 func (c *Concentrator) flushNow(now int64) []model.StatsBucket {
 	var sb []model.StatsBucket
+	var (
+		lenDistributions    int
+		lenErrDistributions int
+	)
 
 	c.mu.Lock()
 	for ts, srb := range c.buckets {
@@ -156,15 +160,18 @@ func (c *Concentrator) flushNow(now int64) []model.StatsBucket {
 
 		log.Debugf("flushing bucket %d", ts)
 		for _, d := range bucket.Distributions {
-			statsd.Client.Histogram("datadog.trace_agent.distribution.len", float64(d.Summary.N), nil, 1)
+			lenDistributions += d.Summary.N
 		}
 		for _, d := range bucket.ErrDistributions {
-			statsd.Client.Histogram("datadog.trace_agent.err_distribution.len", float64(d.Summary.N), nil, 1)
+			lenErrDistributions += d.Summary.N
 		}
 		sb = append(sb, bucket)
 		delete(c.buckets, ts)
 	}
 
+	statsd.Client.Histogram("datadog.trace_agent.distribution.len", float64(lenDistributions), nil, 1)
+	statsd.Client.Histogram("datadog.trace_agent.err_distribution.len", float64(lenErrDistributions), nil, 1)
+
 	// After flushing, update the oldest timestamp allowed to prevent having stats for
 	// an already-flushed bucket.
 	newOldestTs := alignTs(now, c.bsize) - int64(c.bufferLen-1)*c.bsize