Some optimizations for NewSampleTrail with Failed set

name                            old time/op    new time/op    delta
NewSampleFromTrail/no_failed-8    1.02µs ± 6%    1.08µs ± 7%     ~     (p=0.095 n=5+5)
NewSampleFromTrail/failed-8       2.22µs ± 3%    1.43µs ± 5%  -35.61%  (p=0.008 n=5+5)

name                            old alloc/op   new alloc/op   delta
NewSampleFromTrail/no_failed-8      336B ± 0%      336B ± 0%     ~     (all equal)
NewSampleFromTrail/failed-8         752B ± 0%      544B ± 0%  -27.66%  (p=0.008 n=5+5)

name                            old allocs/op  new allocs/op  delta
NewSampleFromTrail/no_failed-8      4.00 ± 0%      4.00 ± 0%     ~     (all equal)
NewSampleFromTrail/failed-8         5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.008 n=5+5)

stats/cloud/data.go

@@ -110,31 +110,22 @@ type SampleDataMap struct {
 // NewSampleFromTrail just creates a ready-to-send Sample instance
 // directly from a httpext.Trail.
 func NewSampleFromTrail(trail *httpext.Trail) *Sample {
-	var values map[string]float64
+	length := 8
+	if trail.Failed.Valid {
+		length++
+	}
+
+	values := make(map[string]float64, length)
+	values[metrics.HTTPReqs.Name] = 1
+	values[metrics.HTTPReqDuration.Name] = stats.D(trail.Duration)
+	values[metrics.HTTPReqBlocked.Name] = stats.D(trail.Blocked)
+	values[metrics.HTTPReqConnecting.Name] = stats.D(trail.Connecting)
+	values[metrics.HTTPReqTLSHandshaking.Name] = stats.D(trail.TLSHandshaking)
+	values[metrics.HTTPReqSending.Name] = stats.D(trail.Sending)
+	values[metrics.HTTPReqWaiting.Name] = stats.D(trail.Waiting)
+	values[metrics.HTTPReqReceiving.Name] = stats.D(trail.Receiving)
 	if trail.Failed.Valid { // this is done so the adding of 1 map element doesn't reexpand the map as this is a hotpath
-		values = map[string]float64{
-			metrics.HTTPReqs.Name:              1,
-			metrics.HTTPReqDuration.Name:       stats.D(trail.Duration),
-			metrics.HTTPReqBlocked.Name:        stats.D(trail.Blocked),
-			metrics.HTTPReqConnecting.Name:     stats.D(trail.Connecting),
-			metrics.HTTPReqTLSHandshaking.Name: stats.D(trail.TLSHandshaking),
-			metrics.HTTPReqSending.Name:        stats.D(trail.Sending),
-			metrics.HTTPReqWaiting.Name:        stats.D(trail.Waiting),
-			metrics.HTTPReqReceiving.Name:      stats.D(trail.Receiving),
-
-			metrics.HTTPReqFailed.Name: stats.B(trail.Failed.Bool),
-		}
-	} else {
-		values = map[string]float64{
-			metrics.HTTPReqs.Name:              1,
-			metrics.HTTPReqDuration.Name:       stats.D(trail.Duration),
-			metrics.HTTPReqBlocked.Name:        stats.D(trail.Blocked),
-			metrics.HTTPReqConnecting.Name:     stats.D(trail.Connecting),
-			metrics.HTTPReqTLSHandshaking.Name: stats.D(trail.TLSHandshaking),
-			metrics.HTTPReqSending.Name:        stats.D(trail.Sending),
-			metrics.HTTPReqWaiting.Name:        stats.D(trail.Waiting),
-			metrics.HTTPReqReceiving.Name:      stats.D(trail.Receiving),
-		}
+		values[metrics.HTTPReqFailed.Name] = stats.B(trail.Failed.Bool)
 	}
 	return &Sample{
 		Type:   DataTypeMap,