Fix Prometheus exporter writing bool label values

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs

@@ -222,10 +222,22 @@ public static int WriteLabel(byte[] buffer, int cursor, string labelKey, object
         buffer[cursor++] = unchecked((byte)'"');
 
         // In Prometheus, a label with an empty label value is considered equivalent to a label that does not exist.
-        cursor = WriteLabelValue(buffer, cursor, labelValue?.ToString() ?? string.Empty);
+        cursor = WriteLabelValue(buffer, cursor, GetLabelValueString(labelValue));
         buffer[cursor++] = unchecked((byte)'"');
 
         return cursor;
+
+        static string GetLabelValueString(object labelValue)
+        {
+            // TODO: Attribute values should be written as their JSON representation. Extra logic may need to be added here to correctly convert other .NET types.
+            // More detail: https://github.com/open-telemetry/opentelemetry-dotnet/issues/4822#issuecomment-1707328495
+            if (labelValue is bool b)
+            {
+                return b ? "true" : "false";
+            }
+
+            return labelValue?.ToString() ?? string.Empty;
+        }
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

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

@@ -155,6 +155,33 @@ public void GaugeOneDimension()
             Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
+    [Fact]
+    public void GaugeBoolDimension()
+    {
+        var buffer = new byte[85000];
+        var metrics = new List<Metric>();
+
+        using var meter = new Meter(Utils.GetCurrentMethodName());
+        using var provider = Sdk.CreateMeterProviderBuilder()
+            .AddMeter(meter.Name)
+            .AddInMemoryExporter(metrics)
+            .Build();
+
+        meter.CreateObservableGauge(
+            "test_gauge",
+            () => new Measurement<long>(123, new KeyValuePair<string, object>("tagKey", true)));
+
+        provider.ForceFlush();
+
+        var cursor = WriteMetric(buffer, 0, metrics[0]);
+        Assert.Matches(
+            ("^"
+                + "# TYPE test_gauge gauge\n"
+                + "test_gauge{tagKey='true'} 123 \\d+\n"
+                + "$").Replace('\'', '"'),
+            Encoding.UTF8.GetString(buffer, 0, cursor));
+    }
+
     [Fact]
     public void GaugeDoubleSubnormal()
     {