Reset scale of exponential histogram (#6557)

Co-authored-by: Rajkumar Rangaraj <[email protected]>

Author: hannahhaering

src/OpenTelemetry/CHANGELOG.md

@@ -6,6 +6,10 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Fixed an issue where the Base2 Exponential Bucket Histogram did not reset its
+  scale to 20 after each collection cycle when using delta aggregation temporality.
+  ([#6557](https://github.com/open-telemetry/opentelemetry-dotnet/pull/6557))
+
 ## 1.13.0
 
 Released 2025-Oct-01

src/OpenTelemetry/Metrics/MetricPoint/Base2ExponentialBucketHistogram.cs

@@ -197,10 +197,16 @@ public void Record(double value)
         Debug.Assert(n == 0, "Increment should always succeed after scale down.");
     }
 
-    internal void Reset()
+    internal void Reset(bool isMinMax)
     {
-        // TODO: Determine if this is sufficient for delta temporality.
-        // I'm not sure we should be resetting the scale.
+        if (isMinMax)
+        {
+            this.RunningMin = double.PositiveInfinity;
+            this.RunningMax = double.NegativeInfinity;
+        }
+
+        this.Scale = Metric.DefaultExponentialHistogramMaxScale;
+        this.RunningSum = 0;
         this.ZeroCount = 0;
         this.PositiveBuckets.Reset();
         this.NegativeBuckets.Reset();

src/OpenTelemetry/Metrics/MetricPoint/MetricPoint.cs

@@ -820,8 +820,7 @@ internal void TakeSnapshot(bool outputDelta)
                     if (outputDelta)
                     {
                         this.runningValue.AsLong = 0;
-                        histogram.RunningSum = 0;
-                        histogram.Reset();
+                        histogram.Reset(isMinMax: false);
                     }
 
                     this.MetricPointStatus = MetricPointStatus.NoCollectPending;
@@ -848,10 +847,7 @@ internal void TakeSnapshot(bool outputDelta)
                     if (outputDelta)
                     {
                         this.runningValue.AsLong = 0;
-                        histogram.RunningSum = 0;
-                        histogram.Reset();
-                        histogram.RunningMin = double.PositiveInfinity;
-                        histogram.RunningMax = double.NegativeInfinity;
+                        histogram.Reset(isMinMax: true);
                     }
 
                     this.MetricPointStatus = MetricPointStatus.NoCollectPending;

test/OpenTelemetry.Tests/Metrics/AggregatorTests.cs

@@ -352,10 +352,12 @@ internal void ExponentialHistogramTests(AggregationType aggregationType, Aggrega
         var count = metricPoint.GetHistogramCount();
         var sum = metricPoint.GetHistogramSum();
         var hasMinMax = metricPoint.TryGetHistogramMinMaxValues(out var min, out var max);
+        var firstScale = metricPoint.GetExponentialHistogramData().Scale;
 
         AssertExponentialBucketsAreCorrect(expectedHistogram, metricPoint.GetExponentialHistogramData());
         Assert.Equal(50, sum);
         Assert.Equal(6, count);
+        Assert.True(firstScale <= Metric.DefaultExponentialHistogramMaxScale, $"The first scale value, {firstScale}, is greater than Metric.DefaultExponentialHistogramMaxScale.");
 
         if (aggregationType == AggregationType.Base2ExponentialHistogramWithMinMax)
         {
@@ -373,12 +375,14 @@ internal void ExponentialHistogramTests(AggregationType aggregationType, Aggrega
         count = metricPoint.GetHistogramCount();
         sum = metricPoint.GetHistogramSum();
         hasMinMax = metricPoint.TryGetHistogramMinMaxValues(out min, out max);
+        var secondScale = metricPoint.GetExponentialHistogramData().Scale;
 
         if (aggregationTemporality == AggregationTemporality.Cumulative)
         {
             AssertExponentialBucketsAreCorrect(expectedHistogram, metricPoint.GetExponentialHistogramData());
             Assert.Equal(50, sum);
             Assert.Equal(6, count);
+            Assert.Equal(firstScale, secondScale);
 
             if (aggregationType == AggregationType.Base2ExponentialHistogramWithMinMax)
             {
@@ -393,10 +397,11 @@ internal void ExponentialHistogramTests(AggregationType aggregationType, Aggrega
         }
         else
         {
-            expectedHistogram.Reset();
+            expectedHistogram.Reset(isMinMax: aggregationType == AggregationType.HistogramWithMinMax);
             AssertExponentialBucketsAreCorrect(expectedHistogram, metricPoint.GetExponentialHistogramData());
             Assert.Equal(0, sum);
             Assert.Equal(0, count);
+            Assert.Equal(Metric.DefaultExponentialHistogramMaxScale, secondScale);
 
             if (aggregationType == AggregationType.Base2ExponentialHistogramWithMinMax)
             {