Add otlptext support for exponential histogram (#4642)

* Add otlptext support for exponential histogram

* comment on correctness

internal/otlptext/databuffer.go

@@ -17,6 +17,7 @@ package otlptext // import "go.opentelemetry.io/collector/internal/otlptext"
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"strconv"
 	"strings"
 
@@ -78,6 +79,10 @@ func (b *dataBuffer) logMetricDataPoints(m pdata.Metric) {
 		data := m.Histogram()
 		b.logEntry("     -> AggregationTemporality: %s", data.AggregationTemporality().String())
 		b.logDoubleHistogramDataPoints(data.DataPoints())
+	case pdata.MetricDataTypeExponentialHistogram:
+		data := m.ExponentialHistogram()
+		b.logEntry("     -> AggregationTemporality: %s", data.AggregationTemporality().String())
+		b.logExponentialHistogramDataPoints(data.DataPoints())
 	case pdata.MetricDataTypeSummary:
 		data := m.Summary()
 		b.logDoubleSummaryDataPoints(data.DataPoints())
@@ -128,6 +133,54 @@ func (b *dataBuffer) logDoubleHistogramDataPoints(ps pdata.HistogramDataPointSli
 	}
 }
 
+func (b *dataBuffer) logExponentialHistogramDataPoints(ps pdata.ExponentialHistogramDataPointSlice) {
+	for i := 0; i < ps.Len(); i++ {
+		p := ps.At(i)
+		b.logEntry("ExponentialHistogramDataPoints #%d", i)
+		b.logDataPointAttributes(p.Attributes())
+
+		b.logEntry("StartTimestamp: %s", p.StartTimestamp())
+		b.logEntry("Timestamp: %s", p.Timestamp())
+		b.logEntry("Count: %d", p.Count())
+		b.logEntry("Sum: %f", p.Sum())
+
+		scale := int(p.Scale())
+		factor := math.Ldexp(math.Ln2, -scale)
+		// Note: the equation used here, which is
+		//   math.Exp(index * factor)
+		// reports +Inf as the _lower_ boundary of the bucket nearest
+		// infinity, which is incorrect and can be addressed in various
+		// ways.  The OTel-Go implementation of this histogram pending
+		// in https://github.com/open-telemetry/opentelemetry-go/pull/2393
+		// uses a lookup table for the last finite boundary, which can be
+		// easily computed using `math/big` (for scales up to 20).
+
+		negB := p.Negative().BucketCounts()
+		posB := p.Positive().BucketCounts()
+
+		for i := 0; i < len(negB); i++ {
+			pos := len(negB) - i - 1
+			index := p.Negative().Offset() + int32(pos)
+			count := p.Negative().BucketCounts()[pos]
+			lower := math.Exp(float64(index) * factor)
+			upper := math.Exp(float64(index+1) * factor)
+			b.logEntry("Bucket (%f, %f], Count: %d", -upper, -lower, count)
+		}
+
+		if p.ZeroCount() != 0 {
+			b.logEntry("Bucket [0, 0], Count: %d", p.ZeroCount())
+		}
+
+		for pos := 0; pos < len(posB); pos++ {
+			index := p.Positive().Offset() + int32(pos)
+			count := p.Positive().BucketCounts()[pos]
+			lower := math.Exp(float64(index) * factor)
+			upper := math.Exp(float64(index+1) * factor)
+			b.logEntry("Bucket [%f, %f), Count: %d", lower, upper, count)
+		}
+	}
+}
+
 func (b *dataBuffer) logDoubleSummaryDataPoints(ps pdata.SummaryDataPointSlice) {
 	for i := 0; i < ps.Len(); i++ {
 		p := ps.At(i)

internal/testdata/metric.go

@@ -258,17 +258,43 @@ func initExponentialHistogramMetric(hm pdata.Metric) {
 	initMetricAttributes13(hdp0.Attributes())
 	hdp0.SetStartTimestamp(TestMetricStartTimestamp)
 	hdp0.SetTimestamp(TestMetricTimestamp)
-	hdp0.SetCount(2)
-	hdp0.SetSum(15)
+	hdp0.SetCount(5)
+	hdp0.SetSum(0.15)
 	hdp0.SetZeroCount(1)
-	hdp1 := hdps.AppendEmpty()
+	hdp0.SetScale(1)
+
+	// positive index 1 and 2 are values sqrt(2), 2 at scale 1
+	hdp0.Positive().SetOffset(1)
+	hdp0.Positive().SetBucketCounts([]uint64{1, 1})
+	// negative index -1 and 0 are values -1/sqrt(2), -1 at scale 1
+	hdp0.Negative().SetOffset(-1)
+	hdp0.Negative().SetBucketCounts([]uint64{1, 1})
+
+	// The above will print:
+	// Bucket (-1.414214, -1.000000], Count: 1
+	// Bucket (-1.000000, -0.707107], Count: 1
+	// Bucket [0, 0], Count: 1
+	// Bucket [0.707107, 1.000000), Count: 1
+	// Bucket [1.000000, 1.414214), Count: 1
 
+	hdp1 := hdps.AppendEmpty()
 	initMetricAttributes2(hdp1.Attributes())
 	hdp1.SetStartTimestamp(TestMetricStartTimestamp)
 	hdp1.SetTimestamp(TestMetricTimestamp)
-	hdp1.SetCount(2)
-	hdp1.SetSum(15)
+	hdp1.SetCount(3)
+	hdp1.SetSum(1.25)
 	hdp1.SetZeroCount(1)
+	hdp1.SetScale(-1)
+
+	// index -1 and 0 are values 0.25, 1 at scale -1
+	hdp1.Positive().SetOffset(-1)
+	hdp1.Positive().SetBucketCounts([]uint64{1, 1})
+
+	// The above will print:
+	// Bucket [0, 0], Count: 1
+	// Bucket [0.250000, 1.000000), Count: 1
+	// Bucket [1.000000, 4.000000), Count: 1
+
 	exemplar := hdp1.Exemplars().AppendEmpty()
 	exemplar.SetTimestamp(TestMetricExemplarTimestamp)
 	exemplar.SetDoubleVal(15)