Implement min/max for the expo-histogram (#215)

lightstep/sdk/metric/aggregator/aggregation/aggregation.go

@@ -66,6 +66,8 @@ type (
 		ZeroCount() uint64
 		Positive() Buckets
 		Negative() Buckets
+		Min() number.Number
+		Max() number.Number
 	}
 
 	// Buckets describes a range of consecutive buckets, starting

lightstep/sdk/metric/aggregator/histogram/exponential.go

@@ -121,6 +121,19 @@ func (s *State[N, Traits]) UpdateByIncr(number N, incr uint64) {
 
 	value := float64(number)
 
+	// Maintain min and max
+	if s.count == 0 {
+		s.min = number
+		s.max = number
+	} else {
+		if number < s.min {
+			s.min = number
+		}
+		if number > s.max {
+			s.max = number
+		}
+	}
+
 	// Note: Not checking for overflow here. TODO.
 	s.count += incr
 
@@ -338,6 +351,18 @@ func (s *State[N, Traits]) Merge(o *State[N, Traits]) {
 	s.lock.Lock()
 	defer s.lock.Unlock()
 
+	if s.count == 0 {
+		s.min = o.min
+		s.max = o.max
+	} else if o.count != 0 {
+		if o.min < s.min {
+			s.min = o.min
+		}
+		if o.max > s.max {
+			s.max = o.max
+		}
+	}
+
 	// Note: Not checking for overflow here. TODO.
 	s.sum += o.sum
 	s.count += o.count

lightstep/sdk/metric/aggregator/histogram/exponential_test.go

@@ -765,6 +765,12 @@ func TestAggregatorCopyMove(t *testing.T) {
 	requireEqualValues(t, h2, h3)
 }
 
+func TestAggregatorMinMax(t *testing.T) {
+	h := NewFloat64(aggregator.HistogramConfig{}, 1, 3, 5, 7, 9)
+	require.Equal(t, 1.0, number.ToFloat64(h.Min()))
+	require.Equal(t, 9.0, number.ToFloat64(h.Max()))
+}
+
 // Benchmarks the Update() function for values in the range [1,2)
 func BenchmarkLinear(b *testing.B) {
 	src := rand.NewSource(77777677777)

lightstep/sdk/metric/aggregator/histogram/histogram.go

@@ -55,6 +55,10 @@ type (
 		// zeroCount is incremented by 1 when the measured
 		// value is exactly 0.
 		zeroCount uint64
+		// min is set when count > 0
+		min N
+		// max is set when count > 0
+		max N
 		// positive holds the positive values
 		positive buckets
 		// negative holds the negative values in these buckets
@@ -147,6 +151,18 @@ func (h *State[N, Traits]) Sum() number.Number {
 	return t.ToNumber(h.sum)
 }
 
+// Min implements aggregation.Histogram.
+func (h *State[N, Traits]) Min() number.Number {
+	var t Traits
+	return t.ToNumber(h.min)
+}
+
+// Max implements aggregation.Histogram.
+func (h *State[N, Traits]) Max() number.Number {
+	var t Traits
+	return t.ToNumber(h.max)
+}
+
 // Count implements aggregation.Histogram.
 func (h *State[N, Traits]) Count() uint64 {
 	return h.count
@@ -214,6 +230,8 @@ func (h *State[N, Traits]) clearState() {
 	h.sum = 0
 	h.count = 0
 	h.zeroCount = 0
+	h.min = 0
+	h.max = 0
 	h.mapping, _ = newMapping(logarithm.MaxScale)
 }
 

lightstep/sdk/metric/exporters/otlp/internal/metrictransform/metric.go

@@ -158,6 +158,13 @@ func HistogramPoints(desc *sdkinstrument.Descriptor, points []data.Point) []*met
 		// supposed to drop the sum.
 		sum := hist.Sum().CoerceToFloat64(desc.NumberKind)
 
+		var minp, maxp *float64
+
+		if hist.Count() != 0 {
+			minp = float64Ptr(hist.Min().CoerceToFloat64(desc.NumberKind))
+			maxp = float64Ptr(hist.Max().CoerceToFloat64(desc.NumberKind))
+		}
+
 		results[i] = &metricspb.ExponentialHistogramDataPoint{
 			Attributes:        Attributes(pt.Attributes),
 			StartTimeUnixNano: toNanos(pt.Start),
@@ -166,6 +173,8 @@ func HistogramPoints(desc *sdkinstrument.Descriptor, points []data.Point) []*met
 			Sum:               &sum,
 			ZeroCount:         hist.ZeroCount(),
 			Scale:             hist.Scale(),
+			Min:               minp,
+			Max:               maxp,
 			Positive:          HistogramBuckets(hist.Positive()),
 			Negative:          HistogramBuckets(hist.Negative()),
 		}

lightstep/sdk/metric/exporters/otlp/internal/metrictransform/metric_test.go

@@ -210,7 +210,7 @@ func TestMetricTransform(t *testing.T) {
 						testUnit,
 						expectCumulative,
 						otlptest.HistogramDataPoint(
-							expectAttrs1, startTime, endTime, 7, 3, 0, 0, 0, []uint64{1, 1, 1}, 0, nil,
+							expectAttrs1, startTime, endTime, 7, 3, 0, 1, 4, 0, 0, []uint64{1, 1, 1}, 0, nil,
 						),
 					),
 				),
@@ -247,6 +247,8 @@ func TestMetricTransform(t *testing.T) {
 							10.5,
 							8,
 							2,
+							-2, // min
+							8,  // max
 							0,
 							// positive offset by 1
 							1,

lightstep/sdk/metric/exporters/otlp/internal/otlptest/otlptest.go

@@ -139,7 +139,7 @@ func Gauge(name, desc, unit string, idps ...*metricspb.NumberDataPoint) *metrics
 	}
 }
 
-func HistogramDataPoint(attributes []*commonpb.KeyValue, start, end time.Time, sum float64, count, zeroCount uint64, scale, posOffset int32, posBucketCounts []uint64, negOffset int32, negBucketCounts []uint64) *metricspb.ExponentialHistogramDataPoint {
+func HistogramDataPoint(attributes []*commonpb.KeyValue, start, end time.Time, sum float64, count, zeroCount uint64, min, max float64, scale, posOffset int32, posBucketCounts []uint64, negOffset int32, negBucketCounts []uint64) *metricspb.ExponentialHistogramDataPoint {
 	dp := &metricspb.ExponentialHistogramDataPoint{
 		Attributes:        attributes,
 		StartTimeUnixNano: toNanos(start),
@@ -149,6 +149,13 @@ func HistogramDataPoint(attributes []*commonpb.KeyValue, start, end time.Time, s
 		ZeroCount:         zeroCount,
 		Scale:             scale,
 	}
+	if !math.IsNaN(min) {
+		dp.Min = &min
+	}
+	if !math.IsNaN(max) {
+		dp.Max = &max
+	}
+
 	if posBucketCounts != nil {
 		dp.Positive = &metricspb.ExponentialHistogramDataPoint_Buckets{
 			Offset:       posOffset,