exphist: defer computing count until collect

Author: dashpole

sdk/metric/internal/aggregate/exponential_histogram.go

@@ -32,10 +32,9 @@ type expoHistogramDataPoint[N int64 | float64] struct {
 	attrs attribute.Set
 	res   FilteredExemplarReservoir[N]
 
-	count uint64
-	min   N
-	max   N
-	sum   N
+	min N
+	max N
+	sum N
 
 	maxSize  int
 	noMinMax bool
@@ -74,8 +73,6 @@ func newExpoHistogramDataPoint[N int64 | float64](
 
 // record adds a new measurement to the histogram. It will rescale the buckets if needed.
 func (p *expoHistogramDataPoint[N]) record(v N) {
-	p.count++
-
 	if !p.noMinMax {
 		if v < p.min {
 			p.min = v
@@ -193,6 +190,10 @@ func (p *expoHistogramDataPoint[N]) scaleChange(bin, startBin int32, length int)
 	return count
 }
 
+func (p *expoHistogramDataPoint[N]) count() uint64 {
+	return p.posBuckets.count() + p.negBuckets.count() + p.zeroCount
+}
+
 // expoBuckets is a set of buckets in an exponential histogram.
 type expoBuckets struct {
 	startBin int32
@@ -285,6 +286,14 @@ func (b *expoBuckets) downscale(delta int32) {
 	b.startBin >>= delta
 }
 
+func (b *expoBuckets) count() uint64 {
+	var total uint64
+	for _, count := range b.counts {
+		total += count
+	}
+	return total
+}
+
 // newExponentialHistogram returns an Aggregator that summarizes a set of
 // measurements as an exponential histogram. Each histogram is scoped by attributes
 // and the aggregation cycle the measurements were made in.
@@ -376,7 +385,7 @@ func (e *expoHistogram[N]) delta(
 		hDPts[i].Attributes = val.attrs
 		hDPts[i].StartTime = e.start
 		hDPts[i].Time = t
-		hDPts[i].Count = val.count
+		hDPts[i].Count = val.count()
 		hDPts[i].Scale = val.scale
 		hDPts[i].ZeroCount = val.zeroCount
 		hDPts[i].ZeroThreshold = 0.0
@@ -439,7 +448,7 @@ func (e *expoHistogram[N]) cumulative(
 		hDPts[i].Attributes = val.attrs
 		hDPts[i].StartTime = e.start
 		hDPts[i].Time = t
-		hDPts[i].Count = val.count
+		hDPts[i].Count = val.count()
 		hDPts[i].Scale = val.scale
 		hDPts[i].ZeroCount = val.zeroCount
 		hDPts[i].ZeroThreshold = 0.0

sdk/metric/internal/aggregate/exponential_histogram_test.go

@@ -706,7 +706,6 @@ func TestSubNormal(t *testing.T) {
 	want := &expoHistogramDataPoint[float64]{
 		attrs:   alice,
 		maxSize: 4,
-		count:   3,
 		min:     math.SmallestNonzeroFloat64,
 		max:     math.SmallestNonzeroFloat64,
 		sum:     3 * math.SmallestNonzeroFloat64,