cmd/trace-agent: only export bucket when needed in concentrator

This change moves the `bucket.Export` call to after the precondition
where it is needed in an attempt to save some memory and CPU.

Author: gbbr

cmd/trace-agent/concentrator.go

@@ -144,17 +144,14 @@ func (c *Concentrator) flushNow(now int64) []model.StatsBucket {
 
 	c.mu.Lock()
 	for ts, srb := range c.buckets {
-		bucket := srb.Export()
-
 		// Always keep `bufferLen` buckets (default is 2: current + previous one).
 		// This is a trade-off: we accept slightly late traces (clock skew and stuff)
 		// but we delay flushing by at most `bufferLen` buckets.
 		if ts > now-int64(c.bufferLen)*c.bsize {
 			continue
 		}
-
 		log.Debugf("flushing bucket %d", ts)
-		sb = append(sb, bucket)
+		sb = append(sb, srb.Export())
 		delete(c.buckets, ts)
 	}