[sdk-metrics] Support .NET 9 Synchronous Gauge

src/OpenTelemetry/CHANGELOG.md

@@ -31,6 +31,11 @@ Notes](../../RELEASENOTES.md).
   See [#5854](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5854)
   for details.
 
+* Add support for collecting metrics from
+  [Gauge](https://learn.microsoft.com/dotnet/api/system.diagnostics.metrics.gauge-1)
+  API
+  ([#5867](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5867))
+
 ## 1.9.0
 
 Released 2024-Jun-14

src/OpenTelemetry/Metrics/Metric.cs

@@ -117,6 +117,12 @@ internal Metric(
             aggType = AggregationType.DoubleGauge;
             this.MetricType = MetricType.DoubleGauge;
         }
+        else if (instrumentIdentity.InstrumentType == typeof(Gauge<double>)
+            || instrumentIdentity.InstrumentType == typeof(Gauge<float>))
+        {
+            aggType = AggregationType.DoubleGauge;
+            this.MetricType = MetricType.DoubleGauge;
+        }
         else if (instrumentIdentity.InstrumentType == typeof(ObservableGauge<long>)
             || instrumentIdentity.InstrumentType == typeof(ObservableGauge<int>)
             || instrumentIdentity.InstrumentType == typeof(ObservableGauge<short>)
@@ -125,6 +131,14 @@ internal Metric(
             aggType = AggregationType.LongGauge;
             this.MetricType = MetricType.LongGauge;
         }
+        else if (instrumentIdentity.InstrumentType == typeof(Gauge<long>)
+            || instrumentIdentity.InstrumentType == typeof(Gauge<int>)
+            || instrumentIdentity.InstrumentType == typeof(Gauge<short>)
+            || instrumentIdentity.InstrumentType == typeof(Gauge<byte>))
+        {
+            aggType = AggregationType.LongGauge;
+            this.MetricType = MetricType.LongGauge;
+        }
         else if (instrumentIdentity.IsHistogram)
         {
             var explicitBucketBounds = instrumentIdentity.HistogramBucketBounds;

src/OpenTelemetry/Metrics/Reader/MetricReader.cs

@@ -27,6 +27,7 @@ public abstract partial class MetricReader : IDisposable
 
             // Temporality is not defined for gauges, so this does not really affect anything.
             var type when type == typeof(ObservableGauge<>) => AggregationTemporality.Delta,
+            var type when type == typeof(Gauge<>) => AggregationTemporality.Delta,
 
             var type when type == typeof(UpDownCounter<>) => AggregationTemporality.Cumulative,
             var type when type == typeof(ObservableUpDownCounter<>) => AggregationTemporality.Cumulative,

test/OpenTelemetry.Tests/Metrics/MetricApiTestsBase.cs

@@ -1613,6 +1613,34 @@ public void UnsupportedMetricInstrument()
         Assert.Empty(exportedItems);
     }
 
+    [Fact]
+    public void GaugeIsExportedCorrectly()
+    {
+        var exportedItems = new List<Metric>();
+
+        using var meter = new Meter($"{Utils.GetCurrentMethodName()}");
+
+        using var container = this.BuildMeterProvider(out var meterProvider, builder => builder
+            .AddMeter(meter.Name)
+            .AddInMemoryExporter(exportedItems));
+
+        var gauge = meter.CreateGauge<long>(name: "NoiseLevel", unit: "dB", description: "Background Noise Level");
+        gauge.Record(10);
+        meterProvider.ForceFlush(MaxTimeToAllowForFlush);
+        Assert.Single(exportedItems);
+        var metric = exportedItems[0];
+        Assert.Equal("Background Noise Level", metric.Description);
+        List<MetricPoint> metricPoints = new List<MetricPoint>();
+        foreach (ref readonly var mp in metric.GetMetricPoints())
+        {
+            metricPoints.Add(mp);
+        }
+
+        var histogramPoint = metricPoints[0];
+        var lastValue = metricPoints[0].GetGaugeLastValueLong();
+        Assert.Equal(10, lastValue);
+    }
+
     internal static IConfiguration BuildConfiguration(bool emitOverflowAttribute, bool shouldReclaimUnusedMetricPoints)
     {
         var configurationData = new Dictionary<string, string>();