[Exporter.Prometheus] Fix number formatting

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md

@@ -15,6 +15,14 @@ Notes](../../RELEASENOTES.md).
   blocks, which could stall concurrent scrapes being waited on.
   ([#7571](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7571))
 
+* Fixed metric values and histogram bucket bounds being written with 17
+  significant digits instead of their shortest round-trippable representation.
+  ([#TODO](https://github.com/open-telemetry/opentelemetry-dotnet/pull/TODO))
+
+* Fixed the canonical representation used for histogram `le` and summary
+  `quantile` label values falling back to 17 significant digits incorrectly.
+  ([#TODO](https://github.com/open-telemetry/opentelemetry-dotnet/pull/TODO))
+
 ## 1.17.0-beta.1
 
 Released 2026-Jul-16

src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md

@@ -15,6 +15,14 @@ Notes](../../RELEASENOTES.md).
   blocks, which could stall concurrent scrapes being waited on.
   ([#7571](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7571))
 
+* Fixed metric values and histogram bucket bounds being written with 17
+  significant digits instead of their shortest round-trippable representation.
+  ([#TODO](https://github.com/open-telemetry/opentelemetry-dotnet/pull/TODO))
+
+* Fixed the canonical representation used for histogram `le` and summary
+  `quantile` label values falling back to 17 significant digits incorrectly.
+  ([#TODO](https://github.com/open-telemetry/opentelemetry-dotnet/pull/TODO))
+
 ## 1.17.0-beta.1
 
 Released 2026-Jul-16

test/OpenTelemetry.Exporter.Prometheus.HttpListener.FuzzTests/PrometheusSerializerFuzzTests.cs

@@ -45,9 +45,29 @@ public Property WriteLongMatchesReferenceImplementation() => Prop.ForAll(
         static (value) => SerializeLong(value).SequenceEqual(ReferenceWriteLong(value)));
 
     [Property(MaxTest = MaxTests)]
-    public Property WriteDoubleMatchesReferenceImplementation() => Prop.ForAll(
+    public Property WriteDoubleRoundTripsWithoutPadding() => Prop.ForAll(
         Generators.DoubleArbitrary(),
-        static (value) => SerializeDouble(value).SequenceEqual(ReferenceWriteDouble(value)));
+        static (value) =>
+        {
+            var written = Encoding.UTF8.GetString(SerializeDouble(value));
+
+            if (double.IsNaN(value))
+            {
+                return written == "NaN";
+            }
+            else if (double.IsPositiveInfinity(value))
+            {
+                return written == "+Inf";
+            }
+            else if (double.IsNegativeInfinity(value))
+            {
+                return written == "-Inf";
+            }
+
+            return double.TryParse(written, NumberStyles.Float, CultureInfo.InvariantCulture, out var parsed)
+                && parsed.Equals(value)
+                && written.Length <= value.ToString("G17", CultureInfo.InvariantCulture).Length;
+        });
 
     [Property(MaxTest = MaxTests)]
     public Property EscapeNameWithDotsMatchesReferenceImplementation() => Prop.ForAll(
@@ -207,14 +227,6 @@ c is (>= 'A' and <= 'Z') or
 
     private static byte[] ReferenceWriteLong(long value) => Encoding.UTF8.GetBytes(value.ToString(CultureInfo.InvariantCulture));
 
-    private static byte[] ReferenceWriteDouble(double value) => value switch
-    {
-        var doubleValue when double.IsPositiveInfinity(doubleValue) => Encoding.UTF8.GetBytes("+Inf"),
-        var doubleValue when double.IsNegativeInfinity(doubleValue) => Encoding.UTF8.GetBytes("-Inf"),
-        var doubleValue when double.IsNaN(doubleValue) => Encoding.UTF8.GetBytes("NaN"),
-        _ => Encoding.UTF8.GetBytes(value.ToString("G17", CultureInfo.InvariantCulture)),
-    };
-
     // An independent re-implementation of the dots and values escaping schemes used to validate
     // PrometheusEscaping.EscapeName. It deliberately uses different mechanisms (manual surrogate
     // decoding, a StringBuilder, and int.ToString("x")) than the production code.

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusIntegrationTests.cs

@@ -95,7 +95,9 @@ public async Task Can_Scrape_Prometheus(string accept)
                         histogram.Name,
                         new ExplicitBucketHistogramConfiguration
                         {
-                            Boundaries = [5, 10],
+                            // Boundaries which exercise every shape a bucket bound can be rendered
+                            // with: fixed-point, and both negative and positive scientific notation.
+                            Boundaries = [1.5e-05, 0.005, 5, 10, 1e6],
                             ExemplarReservoirFactory = () => new SimpleFixedSizeExemplarReservoir(3),
                             TagKeys = [KeepTag],
                         })

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs

@@ -82,26 +82,18 @@ public static TheoryData<string, bool> EscapingSchemesAndFormats
         { long.MaxValue, "9223372036854775807" },
         { ulong.MinValue, "0" },
         { ulong.MaxValue, "18446744073709551615" },
-        { float.MinValue, "-3.40282346638528860e+038" },
+        { float.MinValue, "-3.4028234663852886e+38" },
         { 0f, "0.0" },
         { float.NaN, "NaN" },
         { float.NegativeInfinity, "-Inf" },
         { float.PositiveInfinity, "+Inf" },
-        { float.MaxValue, "3.40282346638528860e+038" },
-#if NET
-        { double.MinValue, "-1.79769313486231571e+308" },
-#else
-        { double.MinValue, "-1.79769313486231570e+308" },
-#endif
+        { float.MaxValue, "3.4028234663852886e+38" },
+        { double.MinValue, "-1.7976931348623157e+308" },
         { 0d, "0.0" },
         { double.NegativeInfinity, "-Inf" },
         { double.PositiveInfinity, "+Inf" },
         { double.NaN, "NaN" },
-#if NET
-        { double.MaxValue, "1.79769313486231571e+308" },
-#else
-        { double.MaxValue, "1.79769313486231570e+308" },
-#endif
+        { double.MaxValue, "1.7976931348623157e+308" },
         { decimal.MinValue, "-79228162514264337593543950335" },
         { 0m, "0" },
         { decimal.MaxValue, "79228162514264337593543950335" },
@@ -601,6 +593,37 @@ public async Task HistogramOneDimension()
         await Verify(output, "txt", VerifySettings);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task HistogramWithFractionalBoundaries(bool useOpenMetrics)
+    {
+        var buffer = new byte[85000];
+        var metrics = new List<Metric>();
+
+        using var meter = CreateMeter();
+        using var provider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddView("test_histogram", new ExplicitBucketHistogramConfiguration
+            {
+                // The bucket boundaries used by the ASP.NET Core duration histograms,
+                // none of which are exactly representable as a double.
+                Boundaries = [0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 7.5, 10],
+            })
+            .AddInMemoryExporter(metrics)
+            .Build();
+
+        var histogram = meter.CreateHistogram<double>("test_histogram", unit: "s");
+        histogram.Record(0.0111996);
+
+        provider.ForceFlush();
+
+        var cursor = WriteMetric(buffer, 0, metrics[0], useOpenMetrics);
+        var output = Encoding.UTF8.GetString(buffer, 0, cursor);
+
+        await Verify(output, "txt", VerifySettings).UseParameters(useOpenMetrics);
+    }
+
     [Fact]
     public async Task HistogramTwoDimensions()
     {
@@ -1450,8 +1473,16 @@ public void WriteLongMatchesInvariantFormatting(long value)
     [InlineData(double.NegativeInfinity, "-Inf")]
     [InlineData(-1234.5, "-1234.5")]
     [InlineData(0d, "0")]
+    [InlineData(-0d, "-0")]
     [InlineData(1234.5, "1234.5")]
     [InlineData(double.PositiveInfinity, "+Inf")]
+    //// Values are written using their shortest round-trippable representation, not padded out to
+    //// the 17 significant digits that are only needed to round-trip the least tractable values.
+    [InlineData(0.005d, "0.005")]
+    [InlineData(0.075d, "0.075")]
+    [InlineData(0.0111996d, "0.0111996")]
+    [InlineData(1e-07d, "1E-07")]
+    [InlineData(0.1d + 0.2d, "0.30000000000000004")]
     public void WriteDoubleMatchesInvariantFormatting(double value, string expected)
     {
         var buffer = new byte[64];
@@ -1471,6 +1502,39 @@ public void WriteDoubleFormatsNaN()
         Assert.Equal("NaN", Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
+    [Fact]
+    public void WriteDoubleDoesNotLosePrecision()
+    {
+        // Values whose shortest representation needs 16 or 17 significant digits, which the
+        // default formatting on .NET Framework would silently round to 15 digits.
+        double[] values =
+        [
+            double.Epsilon,
+            double.MinValue,
+            double.MaxValue,
+            0.1d + 0.2d,
+            1d / 3d,
+            Math.PI,
+            2.2250738585072014e-308,
+            7.793112545177861,
+            -6.186496929997836e+44,
+        ];
+
+        var buffer = new byte[64];
+
+        foreach (var value in values)
+        {
+            var cursor = TextFormatSerializer.WriteDouble(buffer, 0, value);
+            var written = Encoding.UTF8.GetString(buffer, 0, cursor);
+
+            Assert.True(
+                double.TryParse(written, NumberStyles.Float, CultureInfo.InvariantCulture, out var parsed),
+                $"'{written}' could not be parsed.");
+
+            Assert.Equal(value, parsed);
+        }
+    }
+
     [Theory]
     [InlineData(false)]
     [InlineData(true)]
@@ -1583,6 +1647,31 @@ public void WriteTargetInfoSkipsEmptyNonSingletonResource(bool openMetricsReques
     [InlineData(1e6d, "1e+06")]
     [InlineData(1e10d, "1e+10")]
     [InlineData(double.PositiveInfinity, "+Inf")]
+    //// The bucket boundaries used by the HTTP metric duration histograms
+    [InlineData(0.005d, "0.005")]
+    [InlineData(0.025d, "0.025")]
+    [InlineData(0.05d, "0.05")]
+    [InlineData(0.075d, "0.075")]
+    [InlineData(0.25d, "0.25")]
+    [InlineData(0.5d, "0.5")]
+    [InlineData(0.75d, "0.75")]
+    [InlineData(2.5d, "2.5")]
+    [InlineData(7.5d, "7.5")]
+    //// Values that are neither a power of ten nor exactly representable, whose
+    //// shortest round-trippable representation has more than three decimal places.
+    [InlineData(0.0025d, "0.0025")]
+    [InlineData(0.0075d, "0.0075")]
+    [InlineData(0.00025d, "0.00025")]
+    [InlineData(1.5e-05d, "1.5e-05")]
+    [InlineData(33.333d, "33.333")]
+    [InlineData(12.5d, "12.5")]
+    [InlineData(1234567.89d, "1.23456789e+06")]
+    [InlineData(0.1d + 0.2d, "0.30000000000000004")]
+    //// Powers of ten outside the range the specification gives examples for
+    [InlineData(1e11d, "1e+11")]
+    [InlineData(1e100d, "1e+100")]
+    [InlineData(1e-100d, "1e-100")]
+    [InlineData(-1e11d, "-1e+11")]
     public void WriteCanonicalLabelValueUsesOpenMetricsCanonicalNumbers(double value, string expected)
     {
         var buffer = new byte[64];
@@ -1602,7 +1691,6 @@ public void WriteCanonicalLabelValueFormatsNaN()
         Assert.Equal("NaN", Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
-#if NET
     [Theory]
     [InlineData(double.PositiveInfinity, 3)]
     [InlineData(0d, 2)]
@@ -1615,19 +1703,31 @@ public void WriteCanonicalLabelValueThrowsArgumentExceptionWhenBufferTooSmall(do
 
         Assert.Equal("Destination buffer too small.", exception.Message);
     }
-#endif
 
+    // The specification only requires consistency for the values it gives examples for, but
+    // every canonical number must still round-trip to the value it was rendered from.
     [Theory]
-    [InlineData(0.00011d, "0.00011")]
-    [InlineData(1e11d, "1.00000000000000000e+011")]
-    [InlineData(1234567.89d, "1.23456788999999990e+006")]
-    public void WriteCanonicalLabelValueUsesBuiltInFormattingForNonCanonicalNumbers(double value, string expected)
+    [InlineData(0.00011d)]
+    [InlineData(1e11d)]
+    [InlineData(1234567.89d)]
+    [InlineData(1000000.0000005d)]
+    [InlineData(double.Epsilon)]
+    [InlineData(double.MinValue)]
+    [InlineData(double.MaxValue)]
+    [InlineData(0.1d + 0.2d)]
+    [InlineData(Math.PI)]
+    public void WriteCanonicalLabelValueRoundTrips(double value)
     {
         var buffer = new byte[64];
 
         var cursor = TextFormatSerializer.WriteCanonicalLabelValue(buffer, 0, value);
+        var written = Encoding.UTF8.GetString(buffer, 0, cursor);
 
-        Assert.Equal(expected, Encoding.UTF8.GetString(buffer, 0, cursor));
+        Assert.True(
+            double.TryParse(written, NumberStyles.Float, CultureInfo.InvariantCulture, out var parsed),
+            $"'{written}' could not be parsed.");
+
+        Assert.Equal(value, parsed);
     }
 
     [Fact]
@@ -1640,7 +1740,7 @@ public void WriteCanonicalLabelValueDoesNotRoundNearPowersOfTen()
         var near = WriteCanonicalLabelValueToString(NearPowerOfTen);
 
         Assert.Equal("1e+06", exact);
-        Assert.Equal(NearPowerOfTen.ToString("e17", CultureInfo.InvariantCulture), near);
+        Assert.Equal("1.0000000000005e+06", near);
         Assert.NotEqual(exact, near);
 
         static string WriteCanonicalLabelValueToString(double value)

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.HistogramWithFractionalBoundaries_useOpenMetrics=False.verified.txt

@@ -0,0 +1,18 @@
+# TYPE test_histogram_seconds histogram
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.005"} 0
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.01"} 0
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.025"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.05"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.075"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.1"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.25"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.75"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="1"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="2.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="7.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="10"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="+Inf"} 1
+test_histogram_seconds_sum{otel_scope_name="HistogramWithFractionalBoundaries"} 0.0111996
+test_histogram_seconds_count{otel_scope_name="HistogramWithFractionalBoundaries"} 1

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/snapshots/PrometheusSerializer.HistogramWithFractionalBoundaries_useOpenMetrics=True.verified.txt

@@ -0,0 +1,20 @@
+# TYPE test_histogram_seconds histogram
+# UNIT test_histogram_seconds seconds
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.005"} 0
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.01"} 0
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.025"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.05"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.075"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.1"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.25"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="0.75"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="1.0"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="2.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="5.0"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="7.5"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="10.0"} 1
+test_histogram_seconds_bucket{otel_scope_name="HistogramWithFractionalBoundaries",le="+Inf"} 1
+test_histogram_seconds_sum{otel_scope_name="HistogramWithFractionalBoundaries"} 0.0111996
+test_histogram_seconds_count{otel_scope_name="HistogramWithFractionalBoundaries"} 1
+test_histogram_seconds_created{otel_scope_name="HistogramWithFractionalBoundaries"} <TIMESTAMP>