Fix SimpleExemplarReservoir for cumulative (#5230)

Co-authored-by: Alan West <[email protected]>

src/OpenTelemetry/CHANGELOG.md

@@ -15,6 +15,10 @@
   state compared to the current activity. For details see:
   [#5136](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5136)
 
+* Fixed an issue where `SimpleExemplarReservoir` was not resetting internal
+  state for cumulative temporality.
+  [#5230](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5230)
+
 ## 1.7.0
 
 Released 2023-Dec-08

src/OpenTelemetry/Metrics/Exemplar/SimpleExemplarReservoir.cs

@@ -57,10 +57,14 @@ public override Exemplar[] Collect(ReadOnlyTagCollection actualTags, bool reset)
             if (reset)
             {
                 this.runningExemplars[i].Timestamp = default;
-                this.measurementsSeen = 0;
             }
         }
 
+        // Reset internal state irrespective of temporality.
+        // This ensures incoming measurements have fair chance
+        // of making it to the reservoir.
+        this.measurementsSeen = 0;
+
         return this.tempExemplars;
     }
 

test/OpenTelemetry.Tests/Metrics/MetricExemplarTests.cs

@@ -19,8 +19,10 @@ public MetricExemplarTests(ITestOutputHelper output)
         this.output = output;
     }
 
-    [Fact]
-    public void TestExemplarsCounter()
+    [Theory]
+    [InlineData(MetricReaderTemporalityPreference.Cumulative)]
+    [InlineData(MetricReaderTemporalityPreference.Delta)]
+    public void TestExemplarsCounter(MetricReaderTemporalityPreference temporality)
     {
         DateTime testStartTime = DateTime.UtcNow;
         var exportedItems = new List<Metric>();
@@ -33,7 +35,7 @@ public void TestExemplarsCounter()
             .SetExemplarFilter(new AlwaysOnExemplarFilter())
             .AddInMemoryExporter(exportedItems, metricReaderOptions =>
             {
-                metricReaderOptions.TemporalityPreference = MetricReaderTemporalityPreference.Delta;
+                metricReaderOptions.TemporalityPreference = temporality;
             }));
 
         var measurementValues = GenerateRandomValues(10);
@@ -48,6 +50,12 @@ public void TestExemplarsCounter()
         Assert.True(metricPoint.Value.StartTime >= testStartTime);
         Assert.True(metricPoint.Value.EndTime != default);
         var exemplars = GetExemplars(metricPoint.Value);
+
+        // TODO: Modify the test to better test cumulative.
+        // In cumulative where SimpleExemplarReservoir's size is
+        // more than the count of new measurements, it is possible
+        // that the exemplar value is for a measurement that was recorded in the prior
+        // cycle. The current ValidateExemplars() does not handle this case.
         ValidateExemplars(exemplars, metricPoint.Value.StartTime, metricPoint.Value.EndTime, measurementValues, false);
 
         exportedItems.Clear();